Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values
From: Andrii Nakryiko <hidden>
Date: 2021-01-21 23:33:35
Also in:
bpf
On Thu, Jan 21, 2021 at 12:25 PM Jiri Olsa [off-list ref] wrote:
For very large ELF objects (with many sections), we could get special value SHN_XINDEX (65535) for symbol's st_shndx. This patch is adding code to detect the optional extended section index table and use it to resolve symbol's section index. Adding elf_symtab__for_each_symbol_index macro that returns symbol's section index and usign it in collect_symbols function. Signed-off-by: Jiri Olsa <jolsa@kernel.org> ---
You missed fixing up collect_function() as well, which is using elf_sym__section(), which doesn't know about extended numbering.
quoted hunk ↗ jump to hunk
btf_encoder.c | 36 ++++++++++++++++++++++++++++++++---- elf_symtab.c | 39 ++++++++++++++++++++++++++++++++++++++- elf_symtab.h | 2 ++ 3 files changed, 72 insertions(+), 5 deletions(-)diff --git a/btf_encoder.c b/btf_encoder.c index 5557c9efd365..6e6f22c438ce 100644 --- a/btf_encoder.c +++ b/btf_encoder.c@@ -585,12 +585,13 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym) return 0; } -static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl) +static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl, + Elf32_Word sym_sec_idx) { if (!fl->mcount_start && !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) { fl->mcount_start = sym->st_value; - fl->mcount_sec_idx = sym->st_shndx; + fl->mcount_sec_idx = sym_sec_idx; } if (!fl->mcount_stop &&@@ -598,9 +599,36 @@ static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl) fl->mcount_stop = sym->st_value; } +static bool elf_sym__get(Elf_Data *syms, Elf_Data *syms_sec_idx_table, + int id, GElf_Sym *sym, Elf32_Word *sym_sec_idx) +{ + if (!gelf_getsym(syms, id, sym)) + return false; + + *sym_sec_idx = sym->st_shndx; + + if (sym->st_shndx == SHN_XINDEX) { + if (!syms_sec_idx_table) + return false; + if (!gelf_getsymshndx(syms, syms_sec_idx_table, + id, sym, sym_sec_idx))
gelf_getsymshndx() is supposed to work even for cases that don't use
extended numbering, so this should work, right?
if (!gelf_getsymshndx(syms, syms_sec_idx_table, id, sym, sym_sec_idx))
return false;
if (sym->st_shndx == SHN_XINDEX)
*sym_sec_idx = sym->st_shndx;
return true;
?
+ return false; + } + + return true; +} + +#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx) \ + for (id = 0, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, \ + id, &sym, &sym_sec_idx); \ + id < symtab->nr_syms; \ + id++, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, \ + id, &sym, &sym_sec_idx))
what do we want to do if elf_sym__get() returns error (false)? We can either stop or ignore that symbol, right? But currently you are returning invalid symbol data. so either for (id = 0; id < symtab->nr_syms && elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, d, &sym, &sym_sec_idx); id++) or for (id = 0; id < symtab->nr_syms; id++) if (elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, d, &sym, &sym_sec_idx)) But the current variant looks broken. Oh, and elf_symtab__for_each_symbol() is similarly broken, can you please fix that as well? And this new macro should probably be in elf_symtab.h, along the elf_symtab__for_each_symbol.
+
static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
{
struct funcs_layout fl = { };
+ Elf32_Word sym_sec_idx;
uint32_t core_id;
GElf_Sym sym;[...]