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s390/stackleak: Fix __stackleak_poison() inline assembly constraint

The __stackleak_poison() inline assembly comes with a "count" operand where
the "d" constraint is used. "count" is used with the exrl instruction and
"d" means that the compiler may allocate any register from 0 to 15.

If the compiler would allocate register 0 then the exrl instruction would
not or the value of "count" into the executed instruction - resulting in a
stackframe which is only partially poisoned.

Use the correct "a" constraint, which excludes register 0 from register
allocation.

Fixes: 2a405f6bb3a5 ("s390/stackleak: provide fast __stackleak_poison() implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/20260302133500.1560531-4-hca@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

authored by

Heiko Carstens and committed by
Vasily Gorbik 674c5ff0 87ff6da3

+1 -1
+1 -1
arch/s390/include/asm/processor.h
··· 159 159 " j 4f\n" 160 160 "3: mvc 8(1,%[addr]),0(%[addr])\n" 161 161 "4:" 162 - : [addr] "+&a" (erase_low), [count] "+&d" (count), [tmp] "=&a" (tmp) 162 + : [addr] "+&a" (erase_low), [count] "+&a" (count), [tmp] "=&a" (tmp) 163 163 : [poison] "d" (poison) 164 164 : "memory", "cc" 165 165 );