mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
synced 2026-11-06 18:57:23 +00:00
No description
- C 96.8%
- Assembly 0.9%
- Rust 0.7%
- Shell 0.6%
- Python 0.5%
- Other 0.3%
6f9bd8ae03
This works around what seems to be an optimization bug in GCC (at least
13.3.0), where it predicts access_ok() to fail despite the hint to the
contrary.
_copy_to_user() contains:
if (access_ok(to, n)) {
instrument_copy_to_user(to, from, n);
n = raw_copy_to_user(to, from, n);
}
Where access_ok() is likely(__access_ok(addr, size)), yet the compiler
emits conditional jumps forward for the case where it succeeds:
<+0>: endbr64
<+4>: mov %rdx,%rcx
<+7>: mov %rdx,%rax
<+10>: xor %edx,%edx
<+12>: add %rdi,%rcx
<+15>: setb %dl
<+18>: movabs $0x123456789abcdef,%r8
<+28>: test %rdx,%rdx
<+31>: jne 0xffffffff81b3b7c6 <_copy_to_user+38>
<+33>: cmp %rcx,%r8
<+36>: jae 0xffffffff81b3b7cb <_copy_to_user+43>
<+38>: jmp 0xffffffff822673e0 <__x86_return_thunk>
<+43>: nop
<+44>: nop
<+45>: nop
<+46>: mov %rax,%rcx
<+49>: rep movsb %ds:(%rsi),%es:(%rdi)
<+51>: nop
<+52>: nop
<+53>: nop
<+54>: mov %rcx,%rax
<+57>: nop
<+58>: nop
<+59>: nop
<+60>: jmp 0xffffffff822673e0 <__x86_return_thunk>
Patching _copy_to_user() to likely() around the access_ok() use does
not change the asm.
However, spelling out the prediction *within* valid_user_address() does the
trick:
<+0>: endbr64
<+4>: xor %eax,%eax
<+6>: mov %rdx,%rcx
<+9>: add %rdi,%rdx
<+12>: setb %al
<+15>: movabs $0x123456789abcdef,%r8
<+25>: test %rax,%rax
<+28>: jne 0xffffffff81b315e6 <_copy_to_user+54>
<+30>: cmp %rdx,%r8
<+33>: jb 0xffffffff81b315e6 <_copy_to_user+54>
<+35>: nop
<+36>: nop
<+37>: nop
<+38>: rep movsb %ds:(%rsi),%es:(%rdi)
<+40>: nop
<+41>: nop
<+42>: nop
<+43>: nop
<+44>: nop
<+45>: nop
<+46>: mov %rcx,%rax
<+49>: jmp 0xffffffff82255ba0 <__x86_return_thunk>
<+54>: mov %rcx,%rax
<+57>: jmp 0xffffffff82255ba0 <__x86_return_thunk>
Since we kinda expect valid_user_address() to be likely anyway,
add the likely() annotation that also happens to work around
this compiler bug.
[ mingo: Moved the unlikely() branch into valid_user_address() & updated the changelog ]
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250401203029.1132135-1-mjguzik@gmail.com
|
History
|
|
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.