Commit 674c5ff0 authored Mar 02, 2026 by Heiko Carstens Committed by Vasily Gorbik Mar 03, 2026

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: 2a405f6b ("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>

parent 87ff6da3

arch/s390/include/asm/processor.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -159,7 +159,7 @@ static __always_inline void __stackleak_poison(unsigned long erase_low,
		" j 4f\n"
		"3: mvc 8(1,%[addr]),0(%[addr])\n"
		"4:"
		: [addr] "+&a" (erase_low), [count] "+&d" (count), [tmp] "=&a" (tmp)
		: [addr] "+&a" (erase_low), [count] "+&a" (count), [tmp] "=&a" (tmp)
		: [poison] "d" (poison)
		: "memory", "cc"
		);