Re: The trouble with __weak and objtool got worse

Re: The trouble with __weak and objtool got worse

[Nick Desaulniers]

On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > Alternatively:
> >
> >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> >
> > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > work, except I'm getting:
> >
> > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > as: unrecognized option '--generate-unused-section-symbols=yes'
> > as: unrecognized option '--generate-unused-section-symbols=yes'
>
> Reading so hard...
>
>  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
>
> seems to actually work.

$ clang hello.c -Wa,-generate-unused-section-symbols=yes
clang-15: error: unsupported argument
'-generate-unused-section-symbols=yes' to option '-Wa,'

:)

I recall LLVM not emitting STT_SECTION symbols being problematic for
objtool in the past.
Old pre-lore archive:
https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
via https://github.com/ClangBuiltLinux/linux/issues/1209.
(It doesn't look too bad for me to implement)
-- 
Thanks,
~Nick Desaulniers

Re: The trouble with __weak and objtool got worse

[Josh Poimboeuf]

On Fri, Apr 15, 2022 at 10:40:22AM -0700, Nick Desaulniers wrote:
> On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > > Alternatively:
> > >
> > >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> > >
> > > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > > work, except I'm getting:
> > >
> > > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > > as: unrecognized option '--generate-unused-section-symbols=yes'
> >
> > Reading so hard...
> >
> >  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
> >
> > seems to actually work.
> 
> $ clang hello.c -Wa,-generate-unused-section-symbols=yes
> clang-15: error: unsupported argument
> '-generate-unused-section-symbols=yes' to option '-Wa,'
> 
> :)
> 
> I recall LLVM not emitting STT_SECTION symbols being problematic for
> objtool in the past.
> Old pre-lore archive:
> https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
> via https://github.com/ClangBuiltLinux/linux/issues/1209.
> (It doesn't look too bad for me to implement)

Right, and that objtool "fix" -- silently falling back to
non-section-symbol relocations instead of erroring out -- is causing
this issue, when it's done for weak symbols.

If LLVM assembler doesn't support this option then we may have to go
with something like this?  I can't seem to recreate so I'm not able to
test.

We'd also need some objtool checks to make sure the non-section-symbol
fallback isn't being done for a weak symbol.  Or just remove the
fallback altogether and force section symbols whereever they're needed,
similar to the below (untested).


diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
index 5a00b8b2cf9f..77040ce575fa 100644
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -52,9 +52,14 @@ struct static_call_site {
 #define __STATIC_CALL_ADDRESSABLE(name) \
 	__ADDRESSABLE(STATIC_CALL_KEY(name))
 
+extern unsigned long __addressable_ip;
+
+#define __STATIC_CALL_SITE_ADDRESSABLE() __addressable_ip = _THIS_IP_;
+
 #define __static_call(name)						\
 ({									\
 	__STATIC_CALL_ADDRESSABLE(name);				\
+	__STATIC_CALL_SITE_ADDRESSABLE();				\
 	__raw_static_call(name);					\
 })
 
diff --git a/kernel/static_call_inline.c b/kernel/static_call_inline.c
index dc5665b62814..f6e3e0463efb 100644
--- a/kernel/static_call_inline.c
+++ b/kernel/static_call_inline.c
@@ -17,6 +17,8 @@ extern struct static_call_tramp_key __start_static_call_tramp_key[],
 
 static bool static_call_initialized;
 
+unsigned long __section(".discard.addressable") __addressable_ip;
+
 /* mutex to protect key modules/sites */
 static DEFINE_MUTEX(static_call_mutex);
 

Re: The trouble with __weak and objtool got worse

[Josh Poimboeuf]

On Fri, Apr 15, 2022 at 11:21:34AM -0700, Josh Poimboeuf wrote:
> On Fri, Apr 15, 2022 at 10:40:22AM -0700, Nick Desaulniers wrote:
> > On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > > > Alternatively:
> > > >
> > > >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> > > >
> > > > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > > > work, except I'm getting:
> > > >
> > > > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > >
> > > Reading so hard...
> > >
> > >  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
> > >
> > > seems to actually work.
> > 
> > $ clang hello.c -Wa,-generate-unused-section-symbols=yes
> > clang-15: error: unsupported argument
> > '-generate-unused-section-symbols=yes' to option '-Wa,'
> > 
> > :)
> > 
> > I recall LLVM not emitting STT_SECTION symbols being problematic for
> > objtool in the past.
> > Old pre-lore archive:
> > https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
> > via https://github.com/ClangBuiltLinux/linux/issues/1209.
> > (It doesn't look too bad for me to implement)
> 
> Right, and that objtool "fix" -- silently falling back to
> non-section-symbol relocations instead of erroring out -- is causing
> this issue, when it's done for weak symbols.
> 
> If LLVM assembler doesn't support this option then we may have to go
> with something like this?  I can't seem to recreate so I'm not able to
> test.
> 
> We'd also need some objtool checks to make sure the non-section-symbol
> fallback isn't being done for a weak symbol.  Or just remove the
> fallback altogether and force section symbols whereever they're needed,
> similar to the below (untested).

(If the patch works, we could hopefully come up with something better,
using asm goto, that doesn't involve actual instructions)

-- 
Josh

Re: The trouble with __weak and objtool got worse

[Nick Desaulniers]

On Fri, Apr 15, 2022 at 11:21 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Fri, Apr 15, 2022 at 10:40:22AM -0700, Nick Desaulniers wrote:
> > On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > > > Alternatively:
> > > >
> > > >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> > > >
> > > > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > > > work, except I'm getting:
> > > >
> > > > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > >
> > > Reading so hard...
> > >
> > >  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
> > >
> > > seems to actually work.
> >
> > $ clang hello.c -Wa,-generate-unused-section-symbols=yes
> > clang-15: error: unsupported argument
> > '-generate-unused-section-symbols=yes' to option '-Wa,'
> >
> > :)
> >
> > I recall LLVM not emitting STT_SECTION symbols being problematic for
> > objtool in the past.
> > Old pre-lore archive:
> > https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
> > via https://github.com/ClangBuiltLinux/linux/issues/1209.
> > (It doesn't look too bad for me to implement)

https://reviews.llvm.org/D123874

>
> Right, and that objtool "fix" -- silently falling back to
> non-section-symbol relocations instead of erroring out -- is causing
> this issue, when it's done for weak symbols.
>
> If LLVM assembler doesn't support this option then we may have to go
> with something like this?  I can't seem to recreate so I'm not able to
> test.

FWIW, I found this note that retaining STT_SECTION will have adverse
binary size effects for LTO.
https://reviews.llvm.org/D93783#2470728
Perhaps food for thought here.
--
Thanks,
~Nick Desaulniers

Re: The trouble with __weak and objtool got worse

[Peter Zijlstra]

On Fri, Apr 15, 2022 at 01:36:12PM -0700, Nick Desaulniers wrote:
> FWIW, I found this note that retaining STT_SECTION will have adverse
> binary size effects for LTO.
> https://reviews.llvm.org/D93783#2470728
> Perhaps food for thought here.

LTO doesn't suffer this problem since then we run objtool after linking
and it never sees the weak stuff to get confused about.

Re: The trouble with __weak and objtool got worse

[Peter Zijlstra]

On Fri, Apr 15, 2022 at 11:21:30AM -0700, Josh Poimboeuf wrote:

> If LLVM assembler doesn't support this option then we may have to go
> with something like this?  I can't seem to recreate so I'm not able to
> test.
> 
> We'd also need some objtool checks to make sure the non-section-symbol
> fallback isn't being done for a weak symbol.  Or just remove the
> fallback altogether and force section symbols whereever they're needed,
> similar to the below (untested).
> 
> 
> diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
> index 5a00b8b2cf9f..77040ce575fa 100644
> --- a/include/linux/static_call_types.h
> +++ b/include/linux/static_call_types.h
> @@ -52,9 +52,14 @@ struct static_call_site {
>  #define __STATIC_CALL_ADDRESSABLE(name) \
>  	__ADDRESSABLE(STATIC_CALL_KEY(name))
>  
> +extern unsigned long __addressable_ip;
> +
> +#define __STATIC_CALL_SITE_ADDRESSABLE() __addressable_ip = _THIS_IP_;
> +
>  #define __static_call(name)						\
>  ({									\
>  	__STATIC_CALL_ADDRESSABLE(name);				\
> +	__STATIC_CALL_SITE_ADDRESSABLE();				\
>  	__raw_static_call(name);					\
>  })
>  
> diff --git a/kernel/static_call_inline.c b/kernel/static_call_inline.c
> index dc5665b62814..f6e3e0463efb 100644
> --- a/kernel/static_call_inline.c
> +++ b/kernel/static_call_inline.c
> @@ -17,6 +17,8 @@ extern struct static_call_tramp_key __start_static_call_tramp_key[],
>  
>  static bool static_call_initialized;
>  
> +unsigned long __section(".discard.addressable") __addressable_ip;
> +
>  /* mutex to protect key modules/sites */
>  static DEFINE_MUTEX(static_call_mutex);

This *might* work for static_call. But what about things like
.retpoline_sites?

If we get an indirect inside the weak function and at
a different place inside the non-weak function, then we end up with two
patch sites in the non-weak function.

And since all of that is compiler generated, we don't have anything to
stick anything in to alleviate trouble.

(in fact, I didn't observe this with static_call, I just picked it for
the example because it was somewhat easier)

Re: The trouble with __weak and objtool got worse

[Josh Poimboeuf]

On Sat, Apr 16, 2022 at 12:48:09PM +0200, Peter Zijlstra wrote:
> This *might* work for static_call. But what about things like
> .retpoline_sites?
> 
> If we get an indirect inside the weak function and at
> a different place inside the non-weak function, then we end up with two
> patch sites in the non-weak function.
> 
> And since all of that is compiler generated, we don't have anything to
> stick anything in to alleviate trouble.
> 
> (in fact, I didn't observe this with static_call, I just picked it for
> the example because it was somewhat easier)

Ok, right.  And ORC entries will be broken.  I found the ones below by
disabling CONFIG_DEBUG_INFO and looking for "sym->bind == STB_WEAK" in
the elf_add_reloc_to_insn() fallback.

I guess objtool will need to bite the bullet and create section symbols.

  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x0: missing section symbol for reference to weak function text
  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x4: missing section symbol for reference to weak function text
  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x8: missing section symbol for reference to weak function text
  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0xc: missing section symbol for reference to weak function text
  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x10: missing section symbol for reference to weak function text
  lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
  lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x0: missing section symbol for reference to weak function text
  lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x4: missing section symbol for reference to weak function text
  lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x10: missing section symbol for reference to weak function text
  lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x18: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x1c: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x20: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x24: missing section symbol for reference to weak function text
  lib/crc32.o: warning: objtool: .orc_unwind_ip+0x28: missing section symbol for reference to weak function text

-- 
Josh

Re: The trouble with __weak and objtool got worse

[H.J. Lu]

On Sat, Apr 16, 2022 at 9:07 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Sat, Apr 16, 2022 at 12:48:09PM +0200, Peter Zijlstra wrote:
> > This *might* work for static_call. But what about things like
> > .retpoline_sites?
> >
> > If we get an indirect inside the weak function and at
> > a different place inside the non-weak function, then we end up with two
> > patch sites in the non-weak function.
> >
> > And since all of that is compiler generated, we don't have anything to
> > stick anything in to alleviate trouble.
> >
> > (in fact, I didn't observe this with static_call, I just picked it for
> > the example because it was somewhat easier)
>
> Ok, right.  And ORC entries will be broken.  I found the ones below by
> disabling CONFIG_DEBUG_INFO and looking for "sym->bind == STB_WEAK" in
> the elf_add_reloc_to_insn() fallback.
>
> I guess objtool will need to bite the bullet and create section symbols.
>
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x0: missing section symbol for reference to weak function text
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x4: missing section symbol for reference to weak function text
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x8: missing section symbol for reference to weak function text
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0xc: missing section symbol for reference to weak function text
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x10: missing section symbol for reference to weak function text
>   lib/clz_ctz.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
>   lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x0: missing section symbol for reference to weak function text
>   lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x4: missing section symbol for reference to weak function text
>   lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x10: missing section symbol for reference to weak function text
>   lib/iomap_copy.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x14: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x18: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x1c: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x20: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x24: missing section symbol for reference to weak function text
>   lib/crc32.o: warning: objtool: .orc_unwind_ip+0x28: missing section symbol for reference to weak function text

STB_WEAK is meaningful when deciding if a symbol definition should
be ignored only when there is no STB_GLOBAL at link-time.   Tools
must keep the section index of a weak symbol definition if there is no
non-weak definition.

-- 
H.J.

Re: The trouble with __weak and objtool got worse

[Peter Zijlstra]

On Sat, Apr 16, 2022 at 09:07:42AM -0700, Josh Poimboeuf wrote:

> I guess objtool will need to bite the bullet and create section symbols.

---
Subject: objtool: Fix relocs vs weak symbols
From: Peter Zijlstra <peterz@infradead.org>
Date: Sun Apr 17 17:03:36 CEST 2022

Occasionally objtool driven code patching (think .static_call_sites
.retpoline_sites etc..) goes sideways and it tries to patch an
instruction that doesn't match.

Much head-scatching and cursing later the problem is as follows:

Consider:

foo-weak.c:

extern void __SCT__foo(void);

__attribute__((weak)) void foo(void)
{
        return __SCT__foo();
}

foo.c:

extern void __SCT__foo(void);
extern void my_foo(void);

void foo(void)
{
        my_foo();
        return __SCT__foo();
}

These generate the obvious code
(gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -c foo*.c):

foo-weak.o:
0000000000000000 <foo>:
   0:   e9 00 00 00 00          jmpq   5 <foo+0x5>      1: R_X86_64_PLT32       __SCT__foo-0x4

foo.o:
0000000000000000 <foo>:
   0:   48 83 ec 08             sub    $0x8,%rsp
   4:   e8 00 00 00 00          callq  9 <foo+0x9>      5: R_X86_64_PLT32       my_foo-0x4
   9:   48 83 c4 08             add    $0x8,%rsp
   d:   e9 00 00 00 00          jmpq   12 <foo+0x12>    e: R_X86_64_PLT32       __SCT__foo-0x4

Now, when we link these two files together, you get something like (ld -r -o foos.o foo-weak.o foo.o):

foos.o:
0000000000000000 <foo-0x10>:
   0:   e9 00 00 00 00          jmpq   5 <foo-0xb>      1: R_X86_64_PLT32       __SCT__foo-0x4
   5:   66 2e 0f 1f 84 00 00 00 00 00   nopw   %cs:0x0(%rax,%rax,1)
   f:   90                      nop

0000000000000010 <foo>:
  10:   48 83 ec 08             sub    $0x8,%rsp
  14:   e8 00 00 00 00          callq  19 <foo+0x9>     15: R_X86_64_PLT32      my_foo-0x4
  19:   48 83 c4 08             add    $0x8,%rsp
  1d:   e9 00 00 00 00          jmpq   22 <foo+0x12>    1e: R_X86_64_PLT32      __SCT__foo-0x4

Noting that ld preserves the weak function text, but strips the symbol
off of it (hence objdump doing that funny negative offset thing). This
does lead to 'interesting' unused code issues with objtool when ran on
linked objects, but that seems to be working (fingers crossed).

So far so good.. Now lets consider the objtool static_call output
section (readelf output, old binutils):

foo-weak.o:

Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000200000002 R_X86_64_PC32          0000000000000000 .text + 0
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

foo.o:

Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000200000002 R_X86_64_PC32          0000000000000000 .text + d
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

foos.o:

Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000100000002 R_X86_64_PC32          0000000000000000 .text + 0
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1
0000000000000008  0000000100000002 R_X86_64_PC32          0000000000000000 .text + 1d
000000000000000c  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

So we have two patch sites, one in the dead code of the weak foo and one
in the real foo. All is well.

*HOWEVER*, sometimes it generates things like this (using new enough
binutils):

foo-weak.o:

Relocation section '.rela.static_call_sites' at offset 0x2c8 contains 1 entry:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000200000002 R_X86_64_PC32          0000000000000000 foo + 0
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

foo.o:

Relocation section '.rela.static_call_sites' at offset 0x310 contains 2 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000200000002 R_X86_64_PC32          0000000000000000 foo + d
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

foos.o:

Relocation section '.rela.static_call_sites' at offset 0x430 contains 4 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000100000002 R_X86_64_PC32          0000000000000000 foo + 0
0000000000000004  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1
0000000000000008  0000000100000002 R_X86_64_PC32          0000000000000000 foo + d
000000000000000c  0000000d00000002 R_X86_64_PC32          0000000000000000 __SCT__foo + 1

And now we can see how that foos.o .static_call_sites goes side-ways, we
now have _two_ patch sites in foo. One for the weak symbol at foo+0
(which is no longer a static_call site!) and one at foo+d which is in
fact the right location.

This seems to happen when objtool cannot find a section symbol, in which
case it falls back to any other symbol to key off of, however in this
case that goes terribly wrong!

As such, teach objtool to create a section symbol when there isn't
one.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 tools/objtool/elf.c |  182 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 160 insertions(+), 22 deletions(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -575,37 +575,175 @@ int elf_add_reloc(struct elf *elf, struc
 	return 0;
 }
 
-int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,
-			  unsigned long offset, unsigned int type,
-			  struct section *insn_sec, unsigned long insn_off)
+/*
+ * Ensure that any reloc section containing references to @sym is marked
+ * changed such that it will get re-generated in elf_rebuild_reloc_sections()
+ * with the new symbol index.
+ */
+static void elf_dirty_reloc_sym(struct elf *elf, struct symbol *sym)
 {
+	struct section *sec;
+
+	list_for_each_entry(sec, &elf->sections, list) {
+		struct reloc *reloc;
+
+		if (sec->changed)
+			continue;
+
+		list_for_each_entry(reloc, &sec->reloc_list, list) {
+			if (reloc->sym == sym) {
+				sec->changed = true;
+				break;
+			}
+		}
+	}
+}
+
+/*
+ * Move the first global symbol, as per sh_info, into a new, higher symbol
+ * index. This fees up the shndx for a new local symbol.
+ */
+static int elf_move_global_symbol(struct elf *elf, struct section *symtab,
+				  struct section *symtab_shndx)
+{
+	Elf_Data *data, *shndx_data = NULL;
+	Elf32_Word first_non_local;
 	struct symbol *sym;
-	int addend;
+	Elf_Scn *s;
 
-	if (insn_sec->sym) {
-		sym = insn_sec->sym;
-		addend = insn_off;
+	first_non_local = symtab->sh.sh_info;
 
-	} else {
-		/*
-		 * The Clang assembler strips section symbols, so we have to
-		 * reference the function symbol instead:
-		 */
-		sym = find_symbol_containing(insn_sec, insn_off);
-		if (!sym) {
-			/*
-			 * Hack alert.  This happens when we need to reference
-			 * the NOP pad insn immediately after the function.
-			 */
-			sym = find_symbol_containing(insn_sec, insn_off - 1);
+	sym = find_symbol_by_index(elf, first_non_local);
+	if (!sym) {
+		WARN("no non-local symbols !?");
+		return first_non_local;
+	}
+
+	s = elf_getscn(elf->elf, symtab->idx);
+	if (!s) {
+		WARN_ELF("elf_getscn");
+		return -1;
+	}
+
+	data = elf_newdata(s);
+	if (!data) {
+		WARN_ELF("elf_newdata");
+		return -1;
+	}
+
+	data->d_buf = &sym->sym;
+	data->d_size = sizeof(sym->sym);
+	data->d_align = 1;
+	data->d_type = ELF_T_SYM;
+
+	sym->idx = symtab->sh.sh_size / sizeof(sym->sym);
+	elf_dirty_reloc_sym(elf, sym);
+
+	symtab->sh.sh_info += 1;
+	symtab->sh.sh_size += data->d_size;
+	symtab->changed = true;
+
+	/* XXX create ? */
+	if (symtab_shndx) {
+		s = elf_getscn(elf->elf, symtab_shndx->idx);
+		if (!s) {
+			WARN_ELF("elf_getscn");
+			return -1;
 		}
 
-		if (!sym) {
-			WARN("can't find symbol containing %s+0x%lx", insn_sec->name, insn_off);
+		shndx_data = elf_newdata(s);
+		if (!shndx_data) {
+			WARN_ELF("elf_newshndx_data");
 			return -1;
 		}
 
-		addend = insn_off - sym->offset;
+		shndx_data->d_buf = &sym->sec->idx;
+		shndx_data->d_size = sizeof(Elf32_Word);
+		shndx_data->d_align = 4;
+		shndx_data->d_type = ELF_T_WORD;
+
+		symtab_shndx->sh.sh_size += 4;
+		symtab_shndx->changed = true;
+	}
+
+	return first_non_local;
+}
+
+static struct symbol *
+elf_create_section_symbol(struct elf *elf, struct section *sec)
+{
+	struct section *symtab, *symtab_shndx;
+	Elf_Data *shndx_data = NULL;
+	struct symbol *sym;
+	Elf32_Word shndx;
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (symtab) {
+		symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
+		if (symtab_shndx)
+			shndx_data = symtab_shndx->data;
+	} else {
+		WARN("no .symtab");
+		return NULL;
+	}
+
+	sym = malloc(sizeof(*sym));
+	if (!sym) {
+		perror("malloc");
+		return NULL;
+	}
+	memset(sym, 0, sizeof(*sym));
+
+	sym->idx = elf_move_global_symbol(elf, symtab, symtab_shndx);
+	if (sym->idx < 0) {
+		WARN("elf_move_global_symbol");
+		return NULL;
+	}
+
+	sym->name = sec->name;
+	sym->sec = sec;
+
+	// st_name 0
+	sym->sym.st_info = GELF_ST_INFO(STB_LOCAL, STT_SECTION);
+	// st_other 0
+	// st_value 0
+	// st_size 0
+	shndx = sec->idx;
+	if (shndx >= SHN_UNDEF && shndx < SHN_LORESERVE) {
+		sym->sym.st_shndx = shndx;
+		if (!shndx_data)
+			shndx = 0;
+	} else {
+		sym->sym.st_shndx = SHN_XINDEX;
+		if (!shndx_data) {
+			WARN("no .symtab_shndx");
+			return NULL;
+		}
+	}
+
+	if (!gelf_update_symshndx(symtab->data, shndx_data, sym->idx, &sym->sym, shndx)) {
+		WARN_ELF("gelf_update_symshndx");
+		return NULL;
+	}
+
+	elf_add_symbol(elf, sym);
+
+	return sym;
+}
+
+int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,
+			  unsigned long offset, unsigned int type,
+			  struct section *insn_sec, unsigned long insn_off)
+{
+	struct symbol *sym = insn_sec->sym;
+	int addend = insn_off;
+
+	if (!sym) {
+		sym = elf_create_section_symbol(elf, insn_sec);
+		if (!sym)
+			return -1;
+
+		insn_sec->sym = sym;
 	}
 
 	return elf_add_reloc(elf, sec, offset, type, sym, addend);

Re: The trouble with __weak and objtool got worse

[Peter Zijlstra]

On Sun, Apr 17, 2022 at 05:44:22PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 16, 2022 at 09:07:42AM -0700, Josh Poimboeuf wrote:
> 
> > I guess objtool will need to bite the bullet and create section symbols.
> 

Which then led to..

I'll properly post the patches on Tue, once this easter thing has run
it's course and we're actually working again, ha!

---
Subject: objtool: Fix type of reloc::addend
From: Peter Zijlstra <peterz@infradead.org>
Date: Sun Apr 17 17:03:40 CEST 2022

Elf{32,64}_Rela::r_addend is of type: Elf{32,64}_Sword, that means
that out reloc::addend needs to be long or face tuncation issues when
we do elf_rebuild_rel_reloc_sections():

  - 107:  48 b8 00 00 00 00 00 00 00 00   movabs $0x0,%rax        109: R_X86_64_64        level4_kernel_pgt+0x80000067
  + 107:  48 b8 00 00 00 00 00 00 00 00   movabs $0x0,%rax        109: R_X86_64_64        level4_kernel_pgt-0x7fffff99

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 tools/objtool/check.c               |    8 ++++----
 tools/objtool/elf.c                 |    2 +-
 tools/objtool/include/objtool/elf.h |    4 ++--
 3 files changed, 7 insertions(+), 7 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -560,12 +560,12 @@ static int add_dead_ends(struct objtool_
 		else if (reloc->addend == reloc->sym->sec->sh.sh_size) {
 			insn = find_last_insn(file, reloc->sym->sec);
 			if (!insn) {
-				WARN("can't find unreachable insn at %s+0x%x",
+				WARN("can't find unreachable insn at %s+0x%lx",
 				     reloc->sym->sec->name, reloc->addend);
 				return -1;
 			}
 		} else {
-			WARN("can't find unreachable insn at %s+0x%x",
+			WARN("can't find unreachable insn at %s+0x%lx",
 			     reloc->sym->sec->name, reloc->addend);
 			return -1;
 		}
@@ -595,12 +595,12 @@ static int add_dead_ends(struct objtool_
 		else if (reloc->addend == reloc->sym->sec->sh.sh_size) {
 			insn = find_last_insn(file, reloc->sym->sec);
 			if (!insn) {
-				WARN("can't find reachable insn at %s+0x%x",
+				WARN("can't find reachable insn at %s+0x%lx",
 				     reloc->sym->sec->name, reloc->addend);
 				return -1;
 			}
 		} else {
-			WARN("can't find reachable insn at %s+0x%x",
+			WARN("can't find reachable insn at %s+0x%lx",
 			     reloc->sym->sec->name, reloc->addend);
 			return -1;
 		}
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -546,7 +546,7 @@ static struct section *elf_create_reloc_
 						int reltype);
 
 int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
-		  unsigned int type, struct symbol *sym, int addend)
+		  unsigned int type, struct symbol *sym, long addend)
 {
 	struct reloc *reloc;
 
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -73,7 +73,7 @@ struct reloc {
 	struct symbol *sym;
 	unsigned long offset;
 	unsigned int type;
-	int addend;
+	long addend;
 	int idx;
 	bool jump_table_start;
 };
@@ -135,7 +135,7 @@ struct elf *elf_open_read(const char *na
 struct section *elf_create_section(struct elf *elf, const char *name, unsigned int sh_flags, size_t entsize, int nr);
 
 int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
-		  unsigned int type, struct symbol *sym, int addend);
+		  unsigned int type, struct symbol *sym, long addend);
 int elf_add_reloc_to_insn(struct elf *elf, struct section *sec,
 			  unsigned long offset, unsigned int type,
 			  struct section *insn_sec, unsigned long insn_off);