[tip: objtool/urgent] objtool: Fix .symtab_shndx handling for elf_create_undef_symbol()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the objtool/urgent branch of tip:

Commit-ID:     584fd3b31889852d0d6f3dd1e3d8e9619b660d2c
Gitweb:        https://git.kernel.org/tip/584fd3b31889852d0d6f3dd1e3d8e9619b660d2c
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 07 Jun 2021 11:45:58 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 10 Jun 2021 10:08:24 +02:00

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: 2f2f7e47f052 ("objtool: Add elf_create_undef_symbol()")
Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/YL3q1qFO9QIRL/BA@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 tools/objtool/elf.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 743c2e9..41bca1d 100644
--- 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);