From: Peter Zijlstra <peterz@infradead.org>
To: "Fāng-ruì Sòng" <maskray@google.com>
Cc: "Nick Desaulniers" <ndesaulniers@google.com>,
"Josh Poimboeuf" <jpoimboe@redhat.com>,
lma@semihalf.com, "Guenter Roeck" <groeck@google.com>,
"Juergen Gross" <jgross@suse.com>,
lb@semihalf.com, LKML <linux-kernel@vger.kernel.org>,
mbenes@suse.com, "Radosław Biernacki" <rad@semihalf.com>,
upstream@semihalf.com,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
"Nathan Chancellor" <nathan@kernel.org>,
"Sami Tolvanen" <samitolvanen@google.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls
Date: Mon, 7 Jun 2021 11:45:58 +0200 [thread overview]
Message-ID: <YL3q1qFO9QIRL/BA@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YL3lQ5QdNV2qwLR/@hirez.programming.kicks-ass.net>
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@infradead.org> 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);
next prev parent reply other threads:[~2021-06-07 9:46 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-26 15:11 [PATCH v3 00/16] x86,objtool: Optimize !RETPOLINE Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 01/16] x86: Add insn_decode_kernel() Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 02/16] x86/alternatives: Optimize optimize_nops() Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:11 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 03/16] x86/retpoline: Simplify retpolines Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-04-06 8:56 ` David Laight
2021-03-26 15:12 ` [PATCH v3 04/16] objtool: Correctly handle retpoline thunk calls Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 05/16] objtool: Per arch retpoline naming Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] objtool: Handle per " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 06/16] objtool: Fix static_call list generation Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 07/16] objtool: Rework rebuild_reloc logic Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` [tip: x86/core] objtool: Rework the elf_rebuild_reloc_section() logic tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 08/16] objtool: Add elf_create_reloc() helper Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 09/16] objtool: Implicitly create reloc sections Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` [tip: x86/core] objtool: Create reloc sections implicitly tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 10/16] objtool: Extract elf_strtab_concat() Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 11/16] objtool: Extract elf_symbol_add() Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 12/16] objtool: Add elf_create_undef_symbol() Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 13/16] objtool: Keep track of retpoline call sites Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 14/16] objtool: Cache instruction relocs Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 15/16] objtool: Skip magical retpoline .altinstr_replacement Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-26 15:12 ` [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls Peter Zijlstra
2021-03-29 16:38 ` Josh Poimboeuf
2021-06-02 15:51 ` Lukasz Majczak
2021-06-02 16:56 ` Peter Zijlstra
2021-06-02 17:10 ` Peter Zijlstra
2021-06-02 20:43 ` Josh Poimboeuf
2021-06-04 20:50 ` Nick Desaulniers
2021-06-04 23:27 ` Nick Desaulniers
2021-06-04 23:50 ` Fangrui Song
2021-06-05 10:38 ` Peter Zijlstra
2021-06-06 1:58 ` Fāng-ruì Sòng
2021-06-07 7:56 ` Peter Zijlstra
2021-06-07 9:22 ` Peter Zijlstra
2021-06-07 9:45 ` Peter Zijlstra [this message]
2021-06-07 17:23 ` Fāng-ruì Sòng
2021-06-07 18:25 ` Peter Zijlstra
2021-06-07 20:54 ` Nick Desaulniers
2021-06-08 9:56 ` Peter Zijlstra
2021-06-08 16:58 ` Nathan Chancellor
2021-06-08 17:22 ` Peter Zijlstra
2021-06-08 17:29 ` Nathan Chancellor
2021-06-08 18:17 ` Peter Zijlstra
2021-06-08 18:49 ` Nathan Chancellor
2021-06-09 7:11 ` Lukasz Majczak
2021-06-09 7:20 ` Peter Zijlstra
2021-06-09 12:23 ` Lukasz Majczak
2021-06-09 15:08 ` Peter Zijlstra
2021-06-09 15:11 ` Peter Zijlstra
2021-06-09 15:56 ` Nathan Chancellor
2021-06-08 18:18 ` Nick Desaulniers
2021-06-07 18:19 ` Peter Zijlstra
2021-06-07 18:27 ` Fāng-ruì Sòng
2021-06-07 18:47 ` Peter Zijlstra
2021-04-01 15:08 ` [tip: x86/core] objtool/x86: " tip-bot2 for Peter Zijlstra
2021-04-03 11:10 ` tip-bot2 for Peter Zijlstra
2021-03-30 15:02 ` [PATCH v3 00/16] x86,objtool: Optimize !RETPOLINE Miroslav Benes
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