linux-kernel - Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86

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Date:   Wed, 13 Sep 2017 18:49:21 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Eugene Syromyatnikov <evgsyr@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number
 when CONFIG_X86_X32=y


On 09/13, Dmitry V. Levin wrote:
>
> Before this change, CONFIG_X86_X32=y fastpath behaviour was different
> from slowpath:

and even with this change they differ if CONFIG_X86_X32=n? do_syscall_64()
does "nr & __SYSCALL_MASK" unconditionally, this clears the upper bits, no?

And why __SYSCALL_MASK is not "unsigned long" ? IOW, why do we want to silently
ignore the upper bits in $rax ?

Or I am totally confused?

Oleg.

> $ 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.
> 
> Fixes: fca460f95e92 ("x32: Handle the x32 system call flag")
> Cc: stable@...r.kernel.org
> Signed-off-by: Dmitry V. Levin <ldv@...linux.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
>  	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
>  	movq	%r10, %rcx
>  
> -- 
> ldv