objtool: Fix .symtab_shndx handling for elf_create_undef_symbol()

When an ELF object uses extended symbol section indexes (IOW it has a
.symtab_shndx section), these must be kept in sync with the regular
symbol table (.symtab).

So for every new symbol we emit, make sure to also emit a
.symtab_shndx value to keep the arrays of equal size.

Note: since we're writing an UNDEF symbol, most GElf_Sym fields will
be 0 and we can repurpose one (st_size) to host the 0 for the xshndx
value.

Fixes: 34ce688a39e9 ("objtool: Add elf_create_undef_symbol()")
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/YL3q1qFO9QIRL/BA@hirez.programming.kicks-ass.net

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 743c2e9d0f564e93596a2fcf3b0ea6dbf348eb6a..41bca1d13d8e40bea0a4bf15402c1389585e4d29 100644 (file)
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -717,7 +717,7 @@ static int elf_add_string(struct elf *elf, struct section *strtab, char *str)
 
 struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
 {
-       struct section *symtab;
+       struct section *symtab, *symtab_shndx;
        struct symbol *sym;
        Elf_Data *data;
        Elf_Scn *s;
@@ -769,6 +769,29 @@ struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
        symtab->len += data->d_size;
        symtab->changed = true;
 
+       symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
+       if (symtab_shndx) {
+               s = elf_getscn(elf->elf, symtab_shndx->idx);
+               if (!s) {
+                       WARN_ELF("elf_getscn");
+                       return NULL;
+               }
+
+               data = elf_newdata(s);
+               if (!data) {
+                       WARN_ELF("elf_newdata");
+                       return NULL;
+               }
+
+               data->d_buf = &sym->sym.st_size; /* conveniently 0 */
+               data->d_size = sizeof(Elf32_Word);
+               data->d_align = 4;
+               data->d_type = ELF_T_WORD;
+
+               symtab_shndx->len += 4;
+               symtab_shndx->changed = true;
+       }
+
        sym->sec = find_section_by_index(elf, 0);
 
        elf_add_symbol(elf, sym);