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btf_encoder: Fix handling of percpu symbols on s390 

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>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Ilya Leoshkevich 2021-10-12 04:26:37 +02:00 committed by Arnaldo Carvalho de Melo
parent 3cde0135ca
commit 16a7acaba4
1 changed files with 14 additions and 7 deletions
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21
btf_encoder.c
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@ -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);