| Message ID | 20201203160245.1014867-9-jackmanb@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | Atomics for eBPF | expand |
On 12/3/20 8:02 AM, Brendan Jackman wrote: > This adds two atomic opcodes, both of which include the BPF_FETCH > flag. XCHG without the BPF_FETCh flag would naturally encode BPF_FETCh => BPF_FETCH > atomic_set. This is not supported because it would be of limited > value to userspace (it doesn't imply any barriers). CMPXCHG without > BPF_FETCH woulud be an atomic compare-and-write. We don't have such > an operation in the kernel so it isn't provided to BPF either. > > There are two significant design decisions made for the CMPXCHG > instruction: > > - To solve the issue that this operation fundamentally has 3 > operands, but we only have two register fields. Therefore the > operand we compare against (the kernel's API calls it 'old') is > hard-coded to be R0. x86 has similar design (and A64 doesn't > have this problem). > > A potential alternative might be to encode the other operand's > register number in the immediate field. > > - The kernel's atomic_cmpxchg returns the old value, while the C11 > userspace APIs return a boolean indicating the comparison > result. Which should BPF do? A64 returns the old value. x86 returns > the old value in the hard-coded register (and also sets a > flag). That means return-old-value is easier to JIT. > > Signed-off-by: Brendan Jackman <jackmanb@google.com> Ack with minor comments in the above and below. Acked-by: Yonghong Song <yhs@fb.com> > Change-Id: I3f19ad867dfd08515eecf72674e6fdefe28424bb > --- > arch/x86/net/bpf_jit_comp.c | 8 ++++++++ > include/linux/filter.h | 20 ++++++++++++++++++++ > include/uapi/linux/bpf.h | 4 +++- > kernel/bpf/core.c | 20 ++++++++++++++++++++ > kernel/bpf/disasm.c | 15 +++++++++++++++ > kernel/bpf/verifier.c | 19 +++++++++++++++++-- > tools/include/linux/filter.h | 20 ++++++++++++++++++++ > tools/include/uapi/linux/bpf.h | 4 +++- > 8 files changed, 106 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > index 88cb09fa3bfb..7d29bc3bb4ff 100644 > --- a/arch/x86/net/bpf_jit_comp.c > +++ b/arch/x86/net/bpf_jit_comp.c > @@ -831,6 +831,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op, > /* src_reg = atomic_fetch_add(*(dst_reg + off), src_reg); */ > EMIT2(0x0F, 0xC1); > break; > + case BPF_XCHG: > + /* src_reg = atomic_xchg(*(u32/u64*)(dst_reg + off), src_reg); */ src_reg = atomic_xchg((u32/u64*)(dst_reg + off), src_reg)? > + EMIT1(0x87); > + break; > + case BPF_CMPXCHG: > + /* r0 = atomic_cmpxchg(*(u32/u64*)(dst_reg + off), r0, src_reg); */ r0 = atomic_cmpxchg((u32/u64*)(dst_reg + off), r0, src_reg)? > + EMIT2(0x0F, 0xB1); > + break; > default: > pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op); > return -EFAULT; > diff --git a/include/linux/filter.h b/include/linux/filter.h > index 4e04d0fc454f..6186280715ed 100644 > --- a/include/linux/filter.h > +++ b/include/linux/filter.h > @@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn) > .off = OFF, \ > .imm = BPF_ADD | BPF_FETCH }) > > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ src_reg = atomic_xchg(dst_reg + off, src_reg)? > + > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ > + ((struct bpf_insn) { \ > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > + .dst_reg = DST, \ > + .src_reg = SRC, \ > + .off = OFF, \ > + .imm = BPF_XCHG }) > + > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)? > + > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ > + ((struct bpf_insn) { \ > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > + .dst_reg = DST, \ > + .src_reg = SRC, \ > + .off = OFF, \ > + .imm = BPF_CMPXCHG }) > + > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ > > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > index 025e377e7229..53334530cc81 100644 > --- a/include/uapi/linux/bpf.h > +++ b/include/uapi/linux/bpf.h > @@ -45,7 +45,9 @@ > #define BPF_EXIT 0x90 /* function return */ > > /* atomic op type fields (stored in immediate) */ > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ > > /* Register numbers */ > enum { > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > index 61e93eb7d363..28f960bc2e30 100644 > --- a/kernel/bpf/core.c > +++ b/kernel/bpf/core.c > @@ -1630,6 +1630,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > (u32) SRC, > (atomic_t *)(unsigned long) (DST + insn->off)); > break; > + case BPF_XCHG: > + SRC = (u32) atomic_xchg( > + (atomic_t *)(unsigned long) (DST + insn->off), > + (u32) SRC); > + break; > + case BPF_CMPXCHG: > + BPF_R0 = (u32) atomic_cmpxchg( > + (atomic_t *)(unsigned long) (DST + insn->off), > + (u32) BPF_R0, (u32) SRC); > + break; > default: > goto default_label; > } > @@ -1647,6 +1657,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > (u64) SRC, > (atomic64_t *)(s64) (DST + insn->off)); > break; > + case BPF_XCHG: > + SRC = (u64) atomic64_xchg( > + (atomic64_t *)(u64) (DST + insn->off), > + (u64) SRC); > + break; > + case BPF_CMPXCHG: > + BPF_R0 = (u64) atomic64_cmpxchg( > + (atomic64_t *)(u64) (DST + insn->off), > + (u64) BPF_R0, (u64) SRC); > + break; > default: > goto default_label; > } > diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c > index 3ee2246a52ef..18357ea9a17d 100644 > --- a/kernel/bpf/disasm.c > +++ b/kernel/bpf/disasm.c > @@ -167,6 +167,21 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs, > BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > insn->dst_reg, insn->off, insn->src_reg); > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && > + insn->imm == BPF_CMPXCHG) { > + verbose(cbs->private_data, "(%02x) r0 = atomic%s_cmpxchg(*(%s *)(r%d %+d), r0, r%d)\n", (%02x) r0 = atomic%s_cmpxchg((%s *)(r%d %+d), r0, r%d)? > + insn->code, > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > + insn->dst_reg, insn->off, > + insn->src_reg); > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && > + insn->imm == BPF_XCHG) { > + verbose(cbs->private_data, "(%02x) r%d = atomic%s_xchg(*(%s *)(r%d %+d), r%d)\n", (%02x) r%d = atomic%s_xchg((%s *)(r%d %+d), r%d)? > + insn->code, insn->src_reg, > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > + insn->dst_reg, insn->off, insn->src_reg); > } else { > verbose(cbs->private_data, "BUG_%02x\n", insn->code); > } > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index a68adbcee370..ccf4315e54e7 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn > static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn) > { > int err; > + int load_reg; nit: not a big deal but maybe put this definition before 'int err' to maintain reverse christmas tree coding style. > > switch (insn->imm) { > case BPF_ADD: > case BPF_ADD | BPF_FETCH: > + case BPF_XCHG: > + case BPF_CMPXCHG: > break; > default: > verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm); > @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > if (err) > return err; > > + if (insn->imm == BPF_CMPXCHG) { > + /* Check comparison of R0 with memory location */ > + err = check_reg_arg(env, BPF_REG_0, SRC_OP); > + if (err) > + return err; > + } > + > if (is_pointer_value(env, insn->src_reg)) { > verbose(env, "R%d leaks addr into mem\n", insn->src_reg); > return -EACCES; > @@ -3656,8 +3666,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > if (!(insn->imm & BPF_FETCH)) > return 0; > > - /* check and record load of old value into src reg */ > - err = check_reg_arg(env, insn->src_reg, DST_OP); > + if (insn->imm == BPF_CMPXCHG) > + load_reg = BPF_REG_0; > + else > + load_reg = insn->src_reg; > + > + /* check and record load of old value */ > + err = check_reg_arg(env, load_reg, DST_OP); > if (err) > return err; > > diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h > index ac7701678e1a..ea99bd17d003 100644 > --- a/tools/include/linux/filter.h > +++ b/tools/include/linux/filter.h > @@ -190,6 +190,26 @@ > .off = OFF, \ > .imm = BPF_ADD | BPF_FETCH }) > > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ src_reg = atomic_xchg(dst_reg + off, src_reg)? > + > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ > + ((struct bpf_insn) { \ > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > + .dst_reg = DST, \ > + .src_reg = SRC, \ > + .off = OFF, \ > + .imm = BPF_XCHG }) > + > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)? > + > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ > + ((struct bpf_insn) { \ > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > + .dst_reg = DST, \ > + .src_reg = SRC, \ > + .off = OFF, \ > + .imm = BPF_CMPXCHG }) > + > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ > > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h > index 025e377e7229..53334530cc81 100644 > --- a/tools/include/uapi/linux/bpf.h > +++ b/tools/include/uapi/linux/bpf.h > @@ -45,7 +45,9 @@ > #define BPF_EXIT 0x90 /* function return */ > > /* atomic op type fields (stored in immediate) */ > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ > > /* Register numbers */ > enum { >
O Thu, Dec 03, 2020 at 09:34:23PM -0800, Yonghong Song wrote: > On 12/3/20 8:02 AM, Brendan Jackman wrote: > > This adds two atomic opcodes, both of which include the BPF_FETCH > > flag. XCHG without the BPF_FETCh flag would naturally encode > > BPF_FETCh => BPF_FETCH Thanks, sorry I think you've already pointed that one out and I didn't fix it! > > atomic_set. This is not supported because it would be of limited > > value to userspace (it doesn't imply any barriers). CMPXCHG without > > BPF_FETCH woulud be an atomic compare-and-write. We don't have such > > an operation in the kernel so it isn't provided to BPF either. > > > > There are two significant design decisions made for the CMPXCHG > > instruction: > > > > - To solve the issue that this operation fundamentally has 3 > > operands, but we only have two register fields. Therefore the > > operand we compare against (the kernel's API calls it 'old') is > > hard-coded to be R0. x86 has similar design (and A64 doesn't > > have this problem). > > > > A potential alternative might be to encode the other operand's > > register number in the immediate field. > > > > - The kernel's atomic_cmpxchg returns the old value, while the C11 > > userspace APIs return a boolean indicating the comparison > > result. Which should BPF do? A64 returns the old value. x86 returns > > the old value in the hard-coded register (and also sets a > > flag). That means return-old-value is easier to JIT. > > > > Signed-off-by: Brendan Jackman <jackmanb@google.com> > > Ack with minor comments in the above and below. Thanks, ack to all the comments. Have run a `grep -r "atomic_.*(\*" *.patch` - hopefully we're now free of this mistake where the first arg is dereferenced in the comments/disasm... > Acked-by: Yonghong Song <yhs@fb.com> > > > Change-Id: I3f19ad867dfd08515eecf72674e6fdefe28424bb > > --- > > arch/x86/net/bpf_jit_comp.c | 8 ++++++++ > > include/linux/filter.h | 20 ++++++++++++++++++++ > > include/uapi/linux/bpf.h | 4 +++- > > kernel/bpf/core.c | 20 ++++++++++++++++++++ > > kernel/bpf/disasm.c | 15 +++++++++++++++ > > kernel/bpf/verifier.c | 19 +++++++++++++++++-- > > tools/include/linux/filter.h | 20 ++++++++++++++++++++ > > tools/include/uapi/linux/bpf.h | 4 +++- > > 8 files changed, 106 insertions(+), 4 deletions(-) > > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > > index 88cb09fa3bfb..7d29bc3bb4ff 100644 > > --- a/arch/x86/net/bpf_jit_comp.c > > +++ b/arch/x86/net/bpf_jit_comp.c > > @@ -831,6 +831,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op, > > /* src_reg = atomic_fetch_add(*(dst_reg + off), src_reg); */ > > EMIT2(0x0F, 0xC1); > > break; > > + case BPF_XCHG: > > + /* src_reg = atomic_xchg(*(u32/u64*)(dst_reg + off), src_reg); */ > > src_reg = atomic_xchg((u32/u64*)(dst_reg + off), src_reg)? > > > + EMIT1(0x87); > > + break; > > + case BPF_CMPXCHG: > > + /* r0 = atomic_cmpxchg(*(u32/u64*)(dst_reg + off), r0, src_reg); */ > > r0 = atomic_cmpxchg((u32/u64*)(dst_reg + off), r0, src_reg)? > > > + EMIT2(0x0F, 0xB1); > > + break; > > default: > > pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op); > > return -EFAULT; > > diff --git a/include/linux/filter.h b/include/linux/filter.h > > index 4e04d0fc454f..6186280715ed 100644 > > --- a/include/linux/filter.h > > +++ b/include/linux/filter.h > > @@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn) > > .off = OFF, \ > > .imm = BPF_ADD | BPF_FETCH }) > > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ > > src_reg = atomic_xchg(dst_reg + off, src_reg)? > > > + > > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ > > + ((struct bpf_insn) { \ > > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > > + .dst_reg = DST, \ > > + .src_reg = SRC, \ > > + .off = OFF, \ > > + .imm = BPF_XCHG }) > > + > > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ > > r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)? > > > + > > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ > > + ((struct bpf_insn) { \ > > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > > + .dst_reg = DST, \ > > + .src_reg = SRC, \ > > + .off = OFF, \ > > + .imm = BPF_CMPXCHG }) > > + > > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ > > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > > index 025e377e7229..53334530cc81 100644 > > --- a/include/uapi/linux/bpf.h > > +++ b/include/uapi/linux/bpf.h > > @@ -45,7 +45,9 @@ > > #define BPF_EXIT 0x90 /* function return */ > > /* atomic op type fields (stored in immediate) */ > > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ > > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ > > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ > > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ > > /* Register numbers */ > > enum { > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > > index 61e93eb7d363..28f960bc2e30 100644 > > --- a/kernel/bpf/core.c > > +++ b/kernel/bpf/core.c > > @@ -1630,6 +1630,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > > (u32) SRC, > > (atomic_t *)(unsigned long) (DST + insn->off)); > > break; > > + case BPF_XCHG: > > + SRC = (u32) atomic_xchg( > > + (atomic_t *)(unsigned long) (DST + insn->off), > > + (u32) SRC); > > + break; > > + case BPF_CMPXCHG: > > + BPF_R0 = (u32) atomic_cmpxchg( > > + (atomic_t *)(unsigned long) (DST + insn->off), > > + (u32) BPF_R0, (u32) SRC); > > + break; > > default: > > goto default_label; > > } > > @@ -1647,6 +1657,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > > (u64) SRC, > > (atomic64_t *)(s64) (DST + insn->off)); > > break; > > + case BPF_XCHG: > > + SRC = (u64) atomic64_xchg( > > + (atomic64_t *)(u64) (DST + insn->off), > > + (u64) SRC); > > + break; > > + case BPF_CMPXCHG: > > + BPF_R0 = (u64) atomic64_cmpxchg( > > + (atomic64_t *)(u64) (DST + insn->off), > > + (u64) BPF_R0, (u64) SRC); > > + break; > > default: > > goto default_label; > > } > > diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c > > index 3ee2246a52ef..18357ea9a17d 100644 > > --- a/kernel/bpf/disasm.c > > +++ b/kernel/bpf/disasm.c > > @@ -167,6 +167,21 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs, > > BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > > bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > > insn->dst_reg, insn->off, insn->src_reg); > > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && > > + insn->imm == BPF_CMPXCHG) { > > + verbose(cbs->private_data, "(%02x) r0 = atomic%s_cmpxchg(*(%s *)(r%d %+d), r0, r%d)\n", > > (%02x) r0 = atomic%s_cmpxchg((%s *)(r%d %+d), r0, r%d)? > > > + insn->code, > > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > > + insn->dst_reg, insn->off, > > + insn->src_reg); > > + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && > > + insn->imm == BPF_XCHG) { > > + verbose(cbs->private_data, "(%02x) r%d = atomic%s_xchg(*(%s *)(r%d %+d), r%d)\n", > > (%02x) r%d = atomic%s_xchg((%s *)(r%d %+d), r%d)? > > > + insn->code, insn->src_reg, > > + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", > > + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], > > + insn->dst_reg, insn->off, insn->src_reg); > > } else { > > verbose(cbs->private_data, "BUG_%02x\n", insn->code); > > } > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index a68adbcee370..ccf4315e54e7 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn > > static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn) > > { > > int err; > > + int load_reg; > > nit: not a big deal but maybe put this definition before 'int err' to > maintain reverse christmas tree coding style. > > > switch (insn->imm) { > > case BPF_ADD: > > case BPF_ADD | BPF_FETCH: > > + case BPF_XCHG: > > + case BPF_CMPXCHG: > > break; > > default: > > verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm); > > @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > > if (err) > > return err; > > + if (insn->imm == BPF_CMPXCHG) { > > + /* Check comparison of R0 with memory location */ > > + err = check_reg_arg(env, BPF_REG_0, SRC_OP); > > + if (err) > > + return err; > > + } > > + > > if (is_pointer_value(env, insn->src_reg)) { > > verbose(env, "R%d leaks addr into mem\n", insn->src_reg); > > return -EACCES; > > @@ -3656,8 +3666,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > > if (!(insn->imm & BPF_FETCH)) > > return 0; > > - /* check and record load of old value into src reg */ > > - err = check_reg_arg(env, insn->src_reg, DST_OP); > > + if (insn->imm == BPF_CMPXCHG) > > + load_reg = BPF_REG_0; > > + else > > + load_reg = insn->src_reg; > > + > > + /* check and record load of old value */ > > + err = check_reg_arg(env, load_reg, DST_OP); > > if (err) > > return err; > > diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h > > index ac7701678e1a..ea99bd17d003 100644 > > --- a/tools/include/linux/filter.h > > +++ b/tools/include/linux/filter.h > > @@ -190,6 +190,26 @@ > > .off = OFF, \ > > .imm = BPF_ADD | BPF_FETCH }) > > +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ > > src_reg = atomic_xchg(dst_reg + off, src_reg)? > > > + > > +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ > > + ((struct bpf_insn) { \ > > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > > + .dst_reg = DST, \ > > + .src_reg = SRC, \ > > + .off = OFF, \ > > + .imm = BPF_XCHG }) > > + > > +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ > > r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg)? > > > + > > +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ > > + ((struct bpf_insn) { \ > > + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ > > + .dst_reg = DST, \ > > + .src_reg = SRC, \ > > + .off = OFF, \ > > + .imm = BPF_CMPXCHG }) > > + > > /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ > > #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ > > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h > > index 025e377e7229..53334530cc81 100644 > > --- a/tools/include/uapi/linux/bpf.h > > +++ b/tools/include/uapi/linux/bpf.h > > @@ -45,7 +45,9 @@ > > #define BPF_EXIT 0x90 /* function return */ > > /* atomic op type fields (stored in immediate) */ > > -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ > > +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ > > +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ > > +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ > > /* Register numbers */ > > enum { > >
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 88cb09fa3bfb..7d29bc3bb4ff 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -831,6 +831,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op, /* src_reg = atomic_fetch_add(*(dst_reg + off), src_reg); */ EMIT2(0x0F, 0xC1); break; + case BPF_XCHG: + /* src_reg = atomic_xchg(*(u32/u64*)(dst_reg + off), src_reg); */ + EMIT1(0x87); + break; + case BPF_CMPXCHG: + /* r0 = atomic_cmpxchg(*(u32/u64*)(dst_reg + off), r0, src_reg); */ + EMIT2(0x0F, 0xB1); + break; default: pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op); return -EFAULT; diff --git a/include/linux/filter.h b/include/linux/filter.h index 4e04d0fc454f..6186280715ed 100644 --- a/include/linux/filter.h +++ b/include/linux/filter.h @@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn) .off = OFF, \ .imm = BPF_ADD | BPF_FETCH }) +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ + +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ + ((struct bpf_insn) { \ + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ + .dst_reg = DST, \ + .src_reg = SRC, \ + .off = OFF, \ + .imm = BPF_XCHG }) + +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ + +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ + ((struct bpf_insn) { \ + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ + .dst_reg = DST, \ + .src_reg = SRC, \ + .off = OFF, \ + .imm = BPF_CMPXCHG }) + /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index 025e377e7229..53334530cc81 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -45,7 +45,9 @@ #define BPF_EXIT 0x90 /* function return */ /* atomic op type fields (stored in immediate) */ -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ /* Register numbers */ enum { diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index 61e93eb7d363..28f960bc2e30 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -1630,6 +1630,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) (u32) SRC, (atomic_t *)(unsigned long) (DST + insn->off)); break; + case BPF_XCHG: + SRC = (u32) atomic_xchg( + (atomic_t *)(unsigned long) (DST + insn->off), + (u32) SRC); + break; + case BPF_CMPXCHG: + BPF_R0 = (u32) atomic_cmpxchg( + (atomic_t *)(unsigned long) (DST + insn->off), + (u32) BPF_R0, (u32) SRC); + break; default: goto default_label; } @@ -1647,6 +1657,16 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) (u64) SRC, (atomic64_t *)(s64) (DST + insn->off)); break; + case BPF_XCHG: + SRC = (u64) atomic64_xchg( + (atomic64_t *)(u64) (DST + insn->off), + (u64) SRC); + break; + case BPF_CMPXCHG: + BPF_R0 = (u64) atomic64_cmpxchg( + (atomic64_t *)(u64) (DST + insn->off), + (u64) BPF_R0, (u64) SRC); + break; default: goto default_label; } diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c index 3ee2246a52ef..18357ea9a17d 100644 --- a/kernel/bpf/disasm.c +++ b/kernel/bpf/disasm.c @@ -167,6 +167,21 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs, BPF_SIZE(insn->code) == BPF_DW ? "64" : "", bpf_ldst_string[BPF_SIZE(insn->code) >> 3], insn->dst_reg, insn->off, insn->src_reg); + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && + insn->imm == BPF_CMPXCHG) { + verbose(cbs->private_data, "(%02x) r0 = atomic%s_cmpxchg(*(%s *)(r%d %+d), r0, r%d)\n", + insn->code, + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], + insn->dst_reg, insn->off, + insn->src_reg); + } else if (BPF_MODE(insn->code) == BPF_ATOMIC && + insn->imm == BPF_XCHG) { + verbose(cbs->private_data, "(%02x) r%d = atomic%s_xchg(*(%s *)(r%d %+d), r%d)\n", + insn->code, insn->src_reg, + BPF_SIZE(insn->code) == BPF_DW ? "64" : "", + bpf_ldst_string[BPF_SIZE(insn->code) >> 3], + insn->dst_reg, insn->off, insn->src_reg); } else { verbose(cbs->private_data, "BUG_%02x\n", insn->code); } diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a68adbcee370..ccf4315e54e7 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn) { int err; + int load_reg; switch (insn->imm) { case BPF_ADD: case BPF_ADD | BPF_FETCH: + case BPF_XCHG: + case BPF_CMPXCHG: break; default: verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm); @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i if (err) return err; + if (insn->imm == BPF_CMPXCHG) { + /* Check comparison of R0 with memory location */ + err = check_reg_arg(env, BPF_REG_0, SRC_OP); + if (err) + return err; + } + if (is_pointer_value(env, insn->src_reg)) { verbose(env, "R%d leaks addr into mem\n", insn->src_reg); return -EACCES; @@ -3656,8 +3666,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i if (!(insn->imm & BPF_FETCH)) return 0; - /* check and record load of old value into src reg */ - err = check_reg_arg(env, insn->src_reg, DST_OP); + if (insn->imm == BPF_CMPXCHG) + load_reg = BPF_REG_0; + else + load_reg = insn->src_reg; + + /* check and record load of old value */ + err = check_reg_arg(env, load_reg, DST_OP); if (err) return err; diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h index ac7701678e1a..ea99bd17d003 100644 --- a/tools/include/linux/filter.h +++ b/tools/include/linux/filter.h @@ -190,6 +190,26 @@ .off = OFF, \ .imm = BPF_ADD | BPF_FETCH }) +/* Atomic exchange, src_reg = atomic_xchg((dst_reg + off), src_reg) */ + +#define BPF_ATOMIC_XCHG(SIZE, DST, SRC, OFF) \ + ((struct bpf_insn) { \ + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ + .dst_reg = DST, \ + .src_reg = SRC, \ + .off = OFF, \ + .imm = BPF_XCHG }) + +/* Atomic compare-exchange, r0 = atomic_cmpxchg((dst_reg + off), r0, src_reg) */ + +#define BPF_ATOMIC_CMPXCHG(SIZE, DST, SRC, OFF) \ + ((struct bpf_insn) { \ + .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \ + .dst_reg = DST, \ + .src_reg = SRC, \ + .off = OFF, \ + .imm = BPF_CMPXCHG }) + /* Memory store, *(uint *) (dst_reg + off16) = imm32 */ #define BPF_ST_MEM(SIZE, DST, OFF, IMM) \ diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h index 025e377e7229..53334530cc81 100644 --- a/tools/include/uapi/linux/bpf.h +++ b/tools/include/uapi/linux/bpf.h @@ -45,7 +45,9 @@ #define BPF_EXIT 0x90 /* function return */ /* atomic op type fields (stored in immediate) */ -#define BPF_FETCH 0x01 /* fetch previous value into src reg */ +#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */ +#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */ +#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */ /* Register numbers */ enum {
This adds two atomic opcodes, both of which include the BPF_FETCH flag. XCHG without the BPF_FETCh flag would naturally encode atomic_set. This is not supported because it would be of limited value to userspace (it doesn't imply any barriers). CMPXCHG without BPF_FETCH woulud be an atomic compare-and-write. We don't have such an operation in the kernel so it isn't provided to BPF either. There are two significant design decisions made for the CMPXCHG instruction: - To solve the issue that this operation fundamentally has 3 operands, but we only have two register fields. Therefore the operand we compare against (the kernel's API calls it 'old') is hard-coded to be R0. x86 has similar design (and A64 doesn't have this problem). A potential alternative might be to encode the other operand's register number in the immediate field. - The kernel's atomic_cmpxchg returns the old value, while the C11 userspace APIs return a boolean indicating the comparison result. Which should BPF do? A64 returns the old value. x86 returns the old value in the hard-coded register (and also sets a flag). That means return-old-value is easier to JIT. Signed-off-by: Brendan Jackman <jackmanb@google.com> Change-Id: I3f19ad867dfd08515eecf72674e6fdefe28424bb --- arch/x86/net/bpf_jit_comp.c | 8 ++++++++ include/linux/filter.h | 20 ++++++++++++++++++++ include/uapi/linux/bpf.h | 4 +++- kernel/bpf/core.c | 20 ++++++++++++++++++++ kernel/bpf/disasm.c | 15 +++++++++++++++ kernel/bpf/verifier.c | 19 +++++++++++++++++-- tools/include/linux/filter.h | 20 ++++++++++++++++++++ tools/include/uapi/linux/bpf.h | 4 +++- 8 files changed, 106 insertions(+), 4 deletions(-)