On Thu, Feb 19, 2015 at 03:43:27PM +0100, Jan Kiszka wrote:
> On 2015-02-19 15:40, Gilles Chanteperdrix wrote:
> > On Thu, Feb 19, 2015 at 03:32:14PM +0100, Jan Kiszka wrote:
> >> On return from exception, we want to check if the current context is
> >> root or head to take the fast exit in the latter case. TIP_HEAD is set
> >> then, thus we have to check for 'ne' (Z==0) after tst.
> >>
> >> This affects only non-legacy users.
> >>
> >> Signed-off-by: Jan Kiszka <[email protected]>
> >> ---
> >>
> >> This is for 3.14, but it should equally well apply to 3.16. Fixes both
> >> sigdebug test as well as the weird sshd deaths on the virtual vexpress
> >> target.
> >>
> >> arch/arm/kernel/entry-armv.S | 4 ++--
> >> 1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> >> index a608340..f6ef839 100644
> >> --- a/arch/arm/kernel/entry-armv.S
> >> +++ b/arch/arm/kernel/entry-armv.S
> >> @@ -736,8 +736,8 @@ ENTRY(ret_from_exception)
> >> get_thread_info tsk
> >> ldr r0, [tsk, #TI_IPIPE]
> >> tst r0, #_TIP_HEAD
> >> - THUMB( it eq)
> >> - beq __ipipe_ret_to_user_irqs_disabled @ Fast exit path over
> >> non-root domains
> >> + THUMB( it ne)
> >> + bne __ipipe_ret_to_user_irqs_disabled @ Fast exit path
> >> over non-root domains
> >
> > Mmm. Looks suspicious. The semantics of the PSR flags with tst is
> > contrary to to what one would believe. No, from compiling a small
> > example, it would seem you are right.
>
> It took me a while to confirm this as well: 'tst' is 'and' without
> writeback. "val & FLAG" gives non-zero if FLAG is set in val. Non-zero
> (Z==0) is tested by 'ne'.
>
> On x86, there are these nice jnz/jz aliases for that purpose.
My check is:
int f(int x)
{
if (x & 1)
return 42;
return -123;
}
Which gives:
00000000 <f>:
0: e3100001 tst r0, #1
4: 03e0007a mvneq r0, #122 ; 0x7a
8: 13a0002a movne r0, #42 ; 0x2a
c: e12fff1e bx lr
I am almost sure I did something similar before wrting the assembly,
well, I screwed up, sorry.
--
Gilles.
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