x86/uaccess: Predict valid_user_address() returning true

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

0 files changed, 0 insertions, 0 deletions