Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values

On Thu, Jan 21, 2021 at 03:32:40PM -0800, Andrii Nakryiko wrote:

SNIP

quoted

@@ -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;

it seems you're right, gelf_getsymshndx seem to work for
both cases, I'll check

if (sym->st_shndx == SHN_XINDEX)
  *sym_sec_idx = sym->st_shndx;

I don't understand this..  gelf_getsymshndx will return both
symbol and proper index, no? also sym_sec_idx is already
assigned from previou call

return true;

?

quoted

+                       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))

if we go ahead with skipping symbols, this one seems good

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.

thanks,
jirka