Messages in this thread | | | Date | Wed, 13 Sep 2017 18:49:21 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] x86/asm/64: do not clear high 32 bits of syscall number when CONFIG_X86_X32=y |
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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@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 > ja 1f /* return -ENOSYS (already in pt_regs->ax) */ > movq %r10, %rcx > > -- > ldv
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