[PATCH] x86: Treat R_386_PLT32 as R_386_PC32

[Fangrui Song]

This is similar to commit b21ebf2fb4cde1618915a97cc773e287ff49173e "x86:
Treat R_X86_64_PLT32 as R_X86_64_PC32", but for i386. As far as Linux
kernel is concerned, R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
requirement that the symbol address is significant.
R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
address significance requirement.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
PLT, but R_386_PLT32 is arguably preferable for -fno-pic code as well:
this can avoid a "canonical PLT entry" (st_shndx=0, st_value!=0) if the
symbol turns out to be defined externally. Latest Clang (since
961f31d8ad14c66829991522d73e14b5a96ff6d4) can use R_386_PLT32 for
compiler produced symbols (if we drop -ffreestanding for CONFIG_X86_32,
library call optimization can produce newer declarations) and future GCC
may use R_386_PLT32 as well if the maintainers agree to adopt an option
like -fdirect-access-external-data to avoid "canonical PLT entry"/copy
relocations https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Reported-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Fangrui Song <mas...@google.com>
---
arch/x86/kernel/module.c | 1 +
arch/x86/tools/relocs.c | 2 ++
2 files changed, 3 insertions(+)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 34b153cbd4ac..5e9a34b5bd74 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -114,6 +114,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
*location += sym->st_value;
break;
case R_386_PC32:
+ case R_386_PLT32:
/* Add the value, subtract its position */
*location += sym->st_value - (uint32_t)location;
break;
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index ce7188cbdae5..717e48ca28b6 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -867,6 +867,7 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
case R_386_PC32:
case R_386_PC16:
case R_386_PC8:
+ case R_386_PLT32:
/*
* NONE can be ignored and PC relative relocations don't
* need to be adjusted.
@@ -910,6 +911,7 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
case R_386_PC32:
case R_386_PC16:
case R_386_PC8:
+ case R_386_PLT32:
/*
* NONE can be ignored and PC relative relocations don't
* need to be adjusted.
--
2.29.2.729.g45daf8777d-goog

[Nick Desaulniers]

On Wed, Jan 6, 2021 at 4:17 PM Fangrui Song <mas...@google.com> wrote:
>
> This is similar to commit b21ebf2fb4cde1618915a97cc773e287ff49173e "x86:
> Treat R_X86_64_PLT32 as R_X86_64_PC32", but for i386. As far as Linux

nit: the format for referring to in tree sha's:

commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32")

ie. `commit <first 12 chars of sha> ("<oneline from commit message>")

> kernel is concerned, R_386_PLT32 can be treated the same as R_386_PC32.
>
> R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
> requirement that the symbol address is significant.
> R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
> address significance requirement.
>
> On x86-64, there is no PIC vs non-PIC PLT distinction and an
> R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
> `call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.
>
> On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
> convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
> PLT, but R_386_PLT32 is arguably preferable for -fno-pic code as well:
> this can avoid a "canonical PLT entry" (st_shndx=0, st_value!=0) if the
> symbol turns out to be defined externally. Latest Clang (since
> 961f31d8ad14c66829991522d73e14b5a96ff6d4) can use R_386_PLT32 for

Is https://reviews.llvm.org/rG37f0c8df47d84ba311fc9a2c1884935ba8961e84
related? If so, that should be linked; it would be good to say
"clang-12" rather than "Latest Clang" since in some time "Latest
Clang" will lose meaning.

> compiler produced symbols (if we drop -ffreestanding for CONFIG_X86_32,
> library call optimization can produce newer declarations) and future GCC
> may use R_386_PLT32 as well if the maintainers agree to adopt an option
> like -fdirect-access-external-data to avoid "canonical PLT entry"/copy
> relocations https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Punctuation for end of sentence.

>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1210
> Reported-by: Arnd Bergmann <ar...@arndb.de>
> Signed-off-by: Fangrui Song <mas...@google.com>

This fixes a build failure for me with clang-12 (ie. top of tree),
thanks for the patch.

Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Tested-by: Nick Desaulniers <ndesau...@google.com>

I also see R_386_PC32 referenced in scripts/mod/modpost.c and wonder
if we'd need to potentially handle R_386_PLT32 relocation types there
as well? No current build failures, so maybe YAGNI.

--
Thanks,
~Nick Desaulniers

[Nathan Chancellor]

I agree with Nick's comments about the commit message. With those
addressed:

Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nathan Chancellor <natecha...@gmail.com>

[Fangrui Song]

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,

R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
requirement that the symbol address is significant.
R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
address significance requirement.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
PLT, but R_386_PLT32 is arguably preferable for -fno-pic code as well:
this can avoid a "canonical PLT entry" (st_shndx=0, st_value!=0) if the
symbol turns out to be defined externally.

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/961f31d8ad14c66829991522d73e14b5a96ff6d4)
can emit R_386_PLT32 for compiler produced symbols (if we drop

-ffreestanding for CONFIG_X86_32, library call optimization can produce

newer declarations) and future GCC -fno-pic may emit R_386_PLT32 as well
if an option like -fno-direct-access-external-data is adopted to avoid
canonical PLT entry/copy relocations.

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Link: https://github.com/llvm/llvm-project/commit/961f31d8ad14c66829991522d73e14b5a96ff6d4
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Reported-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Fangrui Song <mas...@google.com>

Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nick Desaulniers <ndesau...@google.com>

Tested-by: Nathan Chancellor <natecha...@gmail.com>

---

Change in v2:
* Improve commit message

[Fangrui Song]

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,
R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
requirement that the symbol address is significant.
R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
address significance requirement.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC

PLT.

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
can emit R_386_PLT32 for compiler generated function declarations as
well to avoid a canonical PLT entry (st_shndx=0, st_value!=0) if the
symbol turns out to be defined externally. GCC/GNU as will likely keep
using R_386_PC32 because (1) the ABI is legacy (2) the change will drop
a GNU ld non-default visibility ifunc for shared objects.
https://sourceware.org/bugzilla/show_bug.cgi?id=27169

Link: https://github.com/ClangBuiltLinux/linux/issues/1210

Reported-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Fangrui Song <mas...@google.com>
Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nick Desaulniers <ndesau...@google.com>
Tested-by: Nathan Chancellor <natecha...@gmail.com>

---
Change in v2:
* Improve commit message
---

Change in v3:
* Change the GCC link to the more relevant GNU as link.
* Fix the relevant llvm-project commit id.

2.30.0.296.g2bfb1c46d8-goog

[Fāng-ruì Sòng]

Ping:)

[Borislav Petkov]

It's a good thing I have a toolchain guy who can explain to me what you
guys are doing because you need to start writing those commit messages
for !toolchain developers.

On Thu, Jan 14, 2021 at 02:48:19PM -0800, Fangrui Song wrote:
> This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
> R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,
> R_386_PLT32 can be treated the same as R_386_PC32.
>
> R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
> requirement that the symbol address is significant.
> R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
> address significance requirement.

I was told what "significant" means in that context and while it is
clear to you, I'm pretty sure it is not clear to kernel developers who
haven't looked at toolchains in depth. So please elaborate.

> On x86-64, there is no PIC vs non-PIC PLT distinction and an
> R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
> `call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

Also, please explain in short why LLVM is generating R_X86_64_PLT32
relocs now? I.e., is it the same reason as why binutils does that?

I.e., mentioning the big picture of things would help as to why you're
doing this.

> On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
> convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
> PLT.

Convention in general or convention for LLVM?

> clang-12 -fno-pic (since
> https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
> can emit R_386_PLT32 for compiler generated function declarations as
> well to avoid a canonical PLT entry (st_shndx=0, st_value!=0) if the
> symbol turns out to be defined externally. GCC/GNU as will likely keep
> using R_386_PC32 because (1) the ABI is legacy (2) the change will drop
> a GNU ld non-default visibility ifunc for shared objects.
> https://sourceware.org/bugzilla/show_bug.cgi?id=27169

Not sure how useful this paragraph is for kernel developers...

That comment might need adjustment.

> @@ -910,6 +911,7 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
> case R_386_PC32:
> case R_386_PC16:
> case R_386_PC8:
> + case R_386_PLT32:
> /*
> * NONE can be ignored and PC relative relocations don't
> * need to be adjusted.

Ditto.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[Fangrui Song]

On 2021-01-25, Borislav Petkov wrote:
>It's a good thing I have a toolchain guy who can explain to me what you
>guys are doing because you need to start writing those commit messages
>for !toolchain developers.

How about this following message? I'll answer your questions in line as
well. Explaining everything in the message will be quite long... If
someone is interested, I have put every possibly related matter in
https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,
R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types which can
only be used by branches. If the referenced symbol is defined
externally, a PLT will be used.
R_386_PC32/R_X86_64_PC32 are PC-relative relocation types which can be
used by address taking operations and branches. If the referenced symbol
is defined externally, a copy relocation/canonical PLT entry will be
created in the executable.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

This avoids copy relocations/canonical PLT entries.

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the

GCC/GNU as convention is to use R_386_PC32 for non-PIC PLT and
R_386_PLT32 for PIC PLT. Copy relocations/canonical PLT entries are
possible ABI issues but GCC/GNU as will likely keep the status quo

because (1) the ABI is legacy (2) the change will drop a GNU ld

diagnostic for non-default visibility ifunc in shared objects.
https://sourceware.org/bugzilla/show_bug.cgi?id=27169

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)

can emit R_386_PLT32 for compiler generated function declarations,
because preventing canonical PLT entries is weighed over the rare ifunc
diagnostic.

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Reported-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Fangrui Song <mas...@google.com>
Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nick Desaulniers <ndesau...@google.com>
Tested-by: Nathan Chancellor <natecha...@gmail.com>

>On Thu, Jan 14, 2021 at 02:48:19PM -0800, Fangrui Song wrote:
>> This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
>> R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,
>> R_386_PLT32 can be treated the same as R_386_PC32.
>>
>> R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
>> requirement that the symbol address is significant.
>> R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
>> address significance requirement.
>
>I was told what "significant" means in that context and while it is
>clear to you, I'm pretty sure it is not clear to kernel developers who
>haven't looked at toolchains in depth. So please elaborate.

Expanded "significant" to more words. See above.

>> On x86-64, there is no PIC vs non-PIC PLT distinction and an
>> R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
>> `call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.
>
>Also, please explain in short why LLVM is generating R_X86_64_PLT32
>relocs now? I.e., is it the same reason as why binutils does that?
>
>I.e., mentioning the big picture of things would help as to why you're
>doing this.

It has been explained. The LLVM change was in 2018, roughly the same
time when GNU as emitted R_X86_64_PLT32. I think it does not need
extended explanation because of the separate canonical PLT entries
paragraph.

>> On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
>> convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
>> PLT.
>
>Convention in general or convention for LLVM?

Changed to "GCC/GNU as convention".

>> clang-12 -fno-pic (since
>> https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
>> can emit R_386_PLT32 for compiler generated function declarations as
>> well to avoid a canonical PLT entry (st_shndx=0, st_value!=0) if the
>> symbol turns out to be defined externally. GCC/GNU as will likely keep
>> using R_386_PC32 because (1) the ABI is legacy (2) the change will drop
>> a GNU ld non-default visibility ifunc for shared objects.
>> https://sourceware.org/bugzilla/show_bug.cgi?id=27169
>
>Not sure how useful this paragraph is for kernel developers...

Reorganize it a bit...

[Fangrui Song]

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as
R_X86_64_PC32"), but for i386. As far as Linux kernel is concerned,
R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types which can
only be used by branches. If the referenced symbol is defined
externally, a PLT will be used.
R_386_PC32/R_X86_64_PC32 are PC-relative relocation types which can be
used by address taking operations and branches. If the referenced symbol
is defined externally, a copy relocation/canonical PLT entry will be
created in the executable.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

This avoids canonical PLT entries (st_shndx=0, st_value!=0).

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
GCC/GNU as convention is to use R_386_PC32 for non-PIC PLT and
R_386_PLT32 for PIC PLT. Copy relocations/canonical PLT entries are
possible ABI issues but GCC/GNU as will likely keep the status quo
because (1) the ABI is legacy (2) the change will drop a GNU ld
diagnostic for non-default visibility ifunc in shared objects.
https://sourceware.org/bugzilla/show_bug.cgi?id=27169

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
can emit R_386_PLT32 for compiler generated function declarations,
because preventing canonical PLT entries is weighed over the rare ifunc
diagnostic.

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Reported-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Fangrui Song <mas...@google.com>
Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nick Desaulniers <ndesau...@google.com>
Tested-by: Nathan Chancellor <natecha...@gmail.com>

---
Change in v2:
* Improve commit message
---
Change in v3:
* Change the GCC link to the more relevant GNU as link.

* Fix the relevant llvm-project commit.
---
Change in v4:
* Improve comments and commit message
---
arch/x86/kernel/module.c | 1 +
arch/x86/tools/relocs.c | 12 ++++++++----
2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 34b153cbd4ac..5e9a34b5bd74 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -114,6 +114,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
*location += sym->st_value;
break;
case R_386_PC32:
+ case R_386_PLT32:
/* Add the value, subtract its position */
*location += sym->st_value - (uint32_t)location;
break;
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c

index ce7188cbdae5..1c3a1962cade 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -867,9 +867,11 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,

case R_386_PC32:
case R_386_PC16:
case R_386_PC8:
+ case R_386_PLT32:
/*

- * NONE can be ignored and PC relative relocations don't
- * need to be adjusted.
+ * NONE can be ignored and PC relative relocations don't need
+ * to be adjusted. Because sym must be defined, R_386_PLT32 can
+ * be treated the same way as R_386_PC32.
*/
break;

@@ -910,9 +912,11 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,

case R_386_PC32:
case R_386_PC16:
case R_386_PC8:
+ case R_386_PLT32:
/*

- * NONE can be ignored and PC relative relocations don't
- * need to be adjusted.
+ * NONE can be ignored and PC relative relocations don't need
+ * to be adjusted. Because sym must be defined, R_386_PLT32 can
+ * be treated the same way as R_386_PC32.
*/
break;

--
2.30.0.280.ga3ce27912f-goog

[Nick Desaulniers]

Boris, my CI is red since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6
landed (Dec 5) for i386. If you need a shorter (or less toolchain
verbiage) commit message, please consider simply:

```

This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as

R_X86_64_PC32"), but for i386. From that commit message:
As far as the Linux kernel is concerned, R_386_PLT32 can be

treated the same as R_386_PC32.

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)

can emit R_386_PLT32 for compiler generated function declarations.
```

It would help me abuse <strikethrough>alcohol</strikethrough>coffee
less to have one less fire.

--
Thanks,
~Nick Desaulniers

[Sedat Dilek]

Tested-by: Sedat Dilek <sedat...@gmail.com> # v3

- Sedat -

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