From: "Fāng-ruì Sòng" <maskray@google.com>
To: Peter Zijlstra <peterz@infradead.org>
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 10:23:11 -0700 [thread overview]
Message-ID: <20210607172311.ynnrzihgz74vdyjq@google.com> (raw)
In-Reply-To: <YL3q1qFO9QIRL/BA@hirez.programming.kicks-ass.net>
On 2021-06-07, Peter Zijlstra wrote:
>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?
Yes. http://www.sco.com/developers/gabi/latest/ch4.sheader.html says
"Each value corresponds one to one with a symbol table entry and appear in the same order as those entries."
>> 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.
Does the representation use the section index directly? (sym->sym.st_shndx)
This can be fragile when the number of sections changes..., e.g. elf_add_section
So in llvm-objcopy's representation, the section index is represented as
the section object.
struct Symbol {
...
SectionBase *DefinedIn = nullptr;
...
};
In the writer stage, sections are assigned 32-bit indexes and the writer
knows that an SHN_XINDEX for a symbol is needed if the index is >= 0xff00.
>---
> 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 17:23 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
2021-06-07 17:23 ` Fāng-ruì Sòng [this message]
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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