Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 14 Sep 2017 14:05:04 -0700 | 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 Tue, Sep 12, 2017 at 3:57 PM, Dmitry V. Levin <ldv@altlinux.org> 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?
> > 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. Of course, that's slower, but it could be done without performance loss, I think.
--Andy
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