Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

[Peter Zijlstra]

On Mon, Jun 07, 2021 at 11:22:11AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 07, 2021 at 09:56:48AM +0200, Peter Zijlstra wrote:
> > On Sat, Jun 05, 2021 at 06:58:39PM -0700, Fāng-ruì Sòng wrote:
> > > On Sat, Jun 5, 2021 at 3:39 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 
> > > > I think you've absolutely nailed it; but would you have more information
> > > > or a code reference to what you're speaking about? My complete ELF
> > > > and libelf knowledge is very limited and as demonstrated here, I'm not
> > > > at all sure how all that extended index stuff is supposed to work.
> > > 
> > > The section index field of an Elf{32,64}_Sym (st_shndx) is 16-bit, so
> > > it cannot represent a section index greater than 0xffff.
> > > ELF actually reserves values in 0xff00~0xff00 for other purposes, so
> > > st_shndx cannot represent a section whose index is greater or equal to
> > > 0xff00.
> > 
> > Right, that's about as far as I got, but never could find details on how
> > the extension worked in detail, and I clearly muddled it :/
> 
> OK, so I'm all confused again...
> 
> So a .symtab entry has:
> 
> 	st_name  -- strtab offset for the name string
> 	st_value -- where this symbol lives
> 	st_size  -- size of symbol in bytes
> 	st_shndx -- section index to interpret the @st_value above
> 	st_info  -- type+bind
> 	st_other -- visibility
> 
> The thing is, we're adding UNDEF symbols, for the linker to resolve.
> UNDEF has:
> 
> 	st_value := 0
> 	st_size  := 0
> 	st_shndx := 0
> 	st_info  := GLOBAL + NOTYPE
> 	st_other := 0
> 
> Per that, sh_shndx isn't >= SHN_LORESERVE, and I figured we all good.
> 
> 
> Is the problem that .symtab_shndx is expected to contain the exact same
> number of entries as .symtab? And I'm adding to .symtab and not to
> .symtab_shndx, hence getting them out of sync?
> 
> Let me try adding 0s to .symtab_shndx. See if that makes readelf
> happier.

That does indeed seem to do the trick. Bit daft if you ask me, anybody
reading that file ought to have a handy bucket of 0s available, but
whatever.

---
 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 743c2e9d0f56..41bca1d13d8e 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);