diff mbox series

[bpf-next,v6,19/26] bpf: Permit NULL checking pointer with non-zero fixed offset

Message ID 20221111193224.876706-20-memxor@gmail.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series Allocated objects, BPF linked lists | expand

Checks

Context Check Description
netdev/tree_selection success Clearly marked for bpf-next, async
netdev/fixes_present success Fixes tag not required for -next series
netdev/subject_prefix success Link
netdev/cover_letter success Series has a cover letter
netdev/patch_count fail Series longer than 15 patches (and no cover letter)
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 10 this patch: 10
netdev/cc_maintainers warning 8 maintainers not CCed: sdf@google.com kpsingh@kernel.org haoluo@google.com yhs@fb.com jolsa@kernel.org martin.lau@linux.dev song@kernel.org john.fastabend@gmail.com
netdev/build_clang success Errors and warnings before: 5 this patch: 5
netdev/module_param success Was 0 now: 0
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 10 this patch: 10
netdev/checkpatch warning WARNING: line length of 100 exceeds 80 columns WARNING: line length of 108 exceeds 80 columns WARNING: line length of 116 exceeds 80 columns
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
bpf/vmtest-bpf-next-PR success PR summary
bpf/vmtest-bpf-next-VM_Test-4 fail Logs for build for aarch64 with llvm-16
bpf/vmtest-bpf-next-VM_Test-9 success Logs for set-matrix
bpf/vmtest-bpf-next-VM_Test-1 success Logs for ShellCheck
bpf/vmtest-bpf-next-VM_Test-7 success Logs for llvm-toolchain
bpf/vmtest-bpf-next-VM_Test-8 success Logs for set-matrix
bpf/vmtest-bpf-next-VM_Test-5 fail Logs for build for x86_64 with gcc
bpf/vmtest-bpf-next-VM_Test-6 fail Logs for build for x86_64 with llvm-16
bpf/vmtest-bpf-next-VM_Test-2 fail Logs for build for aarch64 with gcc
bpf/vmtest-bpf-next-VM_Test-3 fail Logs for build for aarch64 with llvm-16

Commit Message

Kumar Kartikeya Dwivedi Nov. 11, 2022, 7:32 p.m. UTC 
Pointer increment on seeing PTR_MAYBE_NULL is already protected against,
hence make an exception for PTR_TO_BTF_ID | MEM_ALLOC while still
keeping the warning for other unintended cases that might creep in.

bpf_list_pop_{front,_back} helpers planned to be introduced in next
commit will return a MEM_ALLOC register with incremented offset pointing
to bpf_list_node field. The user is supposed to then obtain the pointer
to the entry using container_of after NULL checking it. The current
restrictions trigger a warning when doing the NULL checking. Revisiting
the reason, it is meant as an assertion which seems to actually work and
catch the bad case.

Hence, under no other circumstances can reg->off be non-zero for a
register that has the PTR_MAYBE_NULL type flag set.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)
diff mbox series

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5f98aa4c9d7c..7f44885d04dc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10791,15 +10791,20 @@  static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 {
 	if (type_may_be_null(reg->type) && reg->id == id &&
 	    !WARN_ON_ONCE(!reg->id)) {
-		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value ||
-				 !tnum_equals_const(reg->var_off, 0) ||
-				 reg->off)) {
+		if (reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0) || reg->off) {
 			/* Old offset (both fixed and variable parts) should
 			 * have been known-zero, because we don't allow pointer
 			 * arithmetic on pointers that might be NULL. If we
 			 * see this happening, don't convert the register.
+			 *
+			 * But in some cases, some helpers that return local
+			 * kptrs advance offset for the returned pointer.
+			 * In those cases, it is fine to expect to see reg->off.
 			 */
-			return;
+			if (WARN_ON_ONCE(reg->type != (PTR_TO_BTF_ID | MEM_ALLOC | PTR_MAYBE_NULL)))
+				return;
+			if (WARN_ON_ONCE(reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0)))
+				return;
 		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;