| Message ID | 20211012022637.399365-1-iii@linux.ibm.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Delegated to: | BPF |
| Headers | show |
| Series | [dwarves] btf_encoder: Fix handling of percpu symbols on s390 | expand |
| Context | Check | Description |
|---|---|---|
| netdev/tree_selection | success | Not a local patch |
Em Tue, Oct 12, 2021 at 04:26:37AM +0200, Ilya Leoshkevich escreveu: > pahole does not generate VARs for percpu symbols on s390. A percpu > symbol definition on a typical x86_64 kernel looks like this: > > [33] .data..percpu PROGBITS 0000000000000000 01c00000 > ^^^^^^^^^^^^^^^^ sh_addr > LOAD 0x0000000001c00000 0x0000000000000000 0x000000000286f000 > ^^^^^^^^^^^^^^^^^^ p_vaddr > 13559: 000000000001ba50 4 OBJECT LOCAL DEFAULT 33 cpu_profile_flip > ^^^^^^^^^^^^^^^^ st_value > > Most importantly, .data..percpu's sh_addr is 0, and this is what pahole > is currently assuming. However, on s390 this is different: > > [37] .data..percpu PROGBITS 00000000019cd000 018ce000 > ^^^^^^^^^^^^^^^^ sh_addr > LOAD 0x000000000136e000 0x000000000146d000 0x000000000146d000 > ^^^^^^^^^^^^^^^^^^ p_vaddr > 80377: 0000000001ba1440 4 OBJECT WEAK DEFAULT 37 cpu_profile_flip > ^^^^^^^^^^^^^^^^ st_value > > Fix by restructuring the code to always use section-relative offsets > for symbols. Change the comment to focus on this invariant. Thanks, applied. - Arnaldo > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> > --- > btf_encoder.c | 21 ++++++++++++++------- > 1 file changed, 14 insertions(+), 7 deletions(-) > > diff --git a/btf_encoder.c b/btf_encoder.c > index c341f95..16e90c3 100644 > --- a/btf_encoder.c > +++ b/btf_encoder.c > @@ -56,7 +56,8 @@ struct btf_encoder { > raw_output, > verbose, > force, > - gen_floats; > + gen_floats, > + is_rel; > uint32_t array_index_id; > struct { > struct var_info vars[MAX_PERCPU_VAR_CNT]; > @@ -1104,6 +1105,13 @@ static int btf_encoder__collect_percpu_var(struct btf_encoder *encoder, GElf_Sym > if (encoder->verbose) > printf("Found per-CPU symbol '%s' at address 0x%" PRIx64 "\n", sym_name, addr); > > + /* Make sure addr is section-relative. For kernel modules (which are > + * ET_REL files) this is already the case. For vmlinux (which is an > + * ET_EXEC file) we need to subtract the section address. > + */ > + if (!encoder->is_rel) > + addr -= encoder->percpu.base_addr; > + > if (encoder->percpu.var_cnt == MAX_PERCPU_VAR_CNT) { > fprintf(stderr, "Reached the limit of per-CPU variables: %d\n", > MAX_PERCPU_VAR_CNT); > @@ -1195,12 +1203,9 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, struct > addr = var->ip.addr; > dwarf_name = variable__name(var); > > - /* DWARF takes into account .data..percpu section offset > - * within its segment, which for vmlinux is 0, but for kernel > - * modules is >0. ELF symbols, on the other hand, don't take > - * into account these offsets (as they are relative to the > - * section start), so to match DWARF and ELF symbols we need > - * to negate the section base address here. > + /* Make sure addr is section-relative. DWARF, unlike ELF, > + * always contains virtual symbol addresses, so subtract > + * the section address unconditionally. > */ > if (addr < encoder->percpu.base_addr || addr >= encoder->percpu.base_addr + encoder->percpu.sec_sz) > continue; > @@ -1322,6 +1327,8 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam > goto out_delete; > } > > + encoder->is_rel = ehdr.e_type == ET_REL; > + > switch (ehdr.e_ident[EI_DATA]) { > case ELFDATA2LSB: > btf__set_endianness(encoder->btf, BTF_LITTLE_ENDIAN); > -- > 2.31.1
diff --git a/btf_encoder.c b/btf_encoder.c index c341f95..16e90c3 100644 --- a/btf_encoder.c +++ b/btf_encoder.c @@ -56,7 +56,8 @@ struct btf_encoder { raw_output, verbose, force, - gen_floats; + gen_floats, + is_rel; uint32_t array_index_id; struct { struct var_info vars[MAX_PERCPU_VAR_CNT]; @@ -1104,6 +1105,13 @@ static int btf_encoder__collect_percpu_var(struct btf_encoder *encoder, GElf_Sym if (encoder->verbose) printf("Found per-CPU symbol '%s' at address 0x%" PRIx64 "\n", sym_name, addr); + /* Make sure addr is section-relative. For kernel modules (which are + * ET_REL files) this is already the case. For vmlinux (which is an + * ET_EXEC file) we need to subtract the section address. + */ + if (!encoder->is_rel) + addr -= encoder->percpu.base_addr; + if (encoder->percpu.var_cnt == MAX_PERCPU_VAR_CNT) { fprintf(stderr, "Reached the limit of per-CPU variables: %d\n", MAX_PERCPU_VAR_CNT); @@ -1195,12 +1203,9 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, struct addr = var->ip.addr; dwarf_name = variable__name(var); - /* DWARF takes into account .data..percpu section offset - * within its segment, which for vmlinux is 0, but for kernel - * modules is >0. ELF symbols, on the other hand, don't take - * into account these offsets (as they are relative to the - * section start), so to match DWARF and ELF symbols we need - * to negate the section base address here. + /* Make sure addr is section-relative. DWARF, unlike ELF, + * always contains virtual symbol addresses, so subtract + * the section address unconditionally. */ if (addr < encoder->percpu.base_addr || addr >= encoder->percpu.base_addr + encoder->percpu.sec_sz) continue; @@ -1322,6 +1327,8 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam goto out_delete; } + encoder->is_rel = ehdr.e_type == ET_REL; + switch (ehdr.e_ident[EI_DATA]) { case ELFDATA2LSB: btf__set_endianness(encoder->btf, BTF_LITTLE_ENDIAN);
pahole does not generate VARs for percpu symbols on s390. A percpu symbol definition on a typical x86_64 kernel looks like this: [33] .data..percpu PROGBITS 0000000000000000 01c00000 ^^^^^^^^^^^^^^^^ sh_addr LOAD 0x0000000001c00000 0x0000000000000000 0x000000000286f000 ^^^^^^^^^^^^^^^^^^ p_vaddr 13559: 000000000001ba50 4 OBJECT LOCAL DEFAULT 33 cpu_profile_flip ^^^^^^^^^^^^^^^^ st_value Most importantly, .data..percpu's sh_addr is 0, and this is what pahole is currently assuming. However, on s390 this is different: [37] .data..percpu PROGBITS 00000000019cd000 018ce000 ^^^^^^^^^^^^^^^^ sh_addr LOAD 0x000000000136e000 0x000000000146d000 0x000000000146d000 ^^^^^^^^^^^^^^^^^^ p_vaddr 80377: 0000000001ba1440 4 OBJECT WEAK DEFAULT 37 cpu_profile_flip ^^^^^^^^^^^^^^^^ st_value Fix by restructuring the code to always use section-relative offsets for symbols. Change the comment to focus on this invariant. Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> --- btf_encoder.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)