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SubjectRe: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86_X32=y
On Thu, Sep 14, 2017 at 2:33 PM, Dmitry V. Levin <ldv@altlinux.org> wrote:
> On Thu, Sep 14, 2017 at 02:05:04PM -0700, Andy Lutomirski wrote:
>> On Tue, Sep 12, 2017 at 3:57 PM, Dmitry V. Levin wrote:
>> > Before this change, CONFIG_X86_X32=y fastpath behaviour was different
>> > from slowpath:
>> >
>> > $ gcc -xc -Wall -O2 - <<'EOF'
>> > #include <unistd.h>
>> > #include <asm/unistd.h>
>> > int main(void) {
>> > unsigned long nr = ~0xffffffffUL | __NR_exit;
>> > return !!syscall(nr, 42, 1, 2, 3, 4, 5);
>> > }
>> > EOF
>> > $ ./a.out; echo \$?=$?
>> > $?=42
>> > $ strace -enone ./a.out
>> > syscall_18446744069414584380(0x2a, 0x1, 0x2, 0x3, 0x4, 0x5) = -1 (errno 38)
>> > +++ exited with 1 +++
>> >
>> > This change syncs CONFIG_X86_X32=y fastpath behaviour with the case
>> > when CONFIG_X86_X32 is not enabled.
>>
>> Do you see real brokenness here, or is it just weird?
>
> It's definitely broken. A syscall should be either valid or invalid
> regardless of implementation peculiarities like fastpath vs slowpath.
>
>> > Fixes: fca460f95e92 ("x32: Handle the x32 system call flag")
>> > Cc: stable@vger.kernel.org
>> > Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
>> > ---
>> > arch/x86/entry/entry_64.S | 8 +++-----
>> > 1 file changed, 3 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> > index 4916725..3bab6af 100644
>> > --- a/arch/x86/entry/entry_64.S
>> > +++ b/arch/x86/entry/entry_64.S
>> > @@ -185,12 +185,10 @@ entry_SYSCALL_64_fastpath:
>> > */
>> > TRACE_IRQS_ON
>> > ENABLE_INTERRUPTS(CLBR_NONE)
>> > -#if __SYSCALL_MASK == ~0
>> > - cmpq $__NR_syscall_max, %rax
>> > -#else
>> > - andl $__SYSCALL_MASK, %eax
>> > - cmpl $__NR_syscall_max, %eax
>> > +#if __SYSCALL_MASK != ~0
>> > + andq $__SYSCALL_MASK, %rax
>> > #endif
>> > + cmpq $__NR_syscall_max, %rax
>>
>> I don't know much about x32 userspace, but there's an argument that
>> the high bits *should* be masked off if the x32 bit is set.
>
> Why?

Because it always worked that way.

That being said, I'd be okay with applying your patch and seeing
whether anything breaks. Ingo?

>
>
> --
> ldv

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