[SRU][J:linux][PATCH 1/1] selftests/powerpc/tm: Fix tcheck() reading uninitialised CR value

[Thibault Ferrante]

BugLink: https://bugs.launchpad.net/bugs/2107442

tcheck() is used to check the current transaction state (active,
suspended, doomed) via the "tcheck" instruction, which writes its
result into CR field 0. The inline asm declared a GPR output operand
for this result but never actually moved the CR into it.

Every caller (tcheck_doomed(), tcheck_active(), tcheck_suspended(),
tcheck_transactional()) has effectively been testing bits of an unrelated,
arbitrary register value since this helper was introduced.
The "& 4" mask discards the TDOOMED and TS_lsb (suspended) bits before
they ever reach the callers, so tcheck_doomed() and tcheck_suspended()
can never return true, and tcheck_transactional() degrades to being
equivalent to tcheck_active().

Fix tcheck() to actually move CR into the output register with mfcr,
and widen the mask from "& 4" to "& 0xf" so the full CR0 nibble
(TDOOMED | TS_msb | TS_lsb | reserved) is preserved for the callers.

This bug has been present since tcheck() was introduced.

Link: https://bugs.launchpad.net/bugs/2107442
Fixes: 8e03bd4e70b6 ("selftests/powerpc: Add TM tcheck helpers in C")
Signed-off-by: Thibault Ferrante <thibault.ferrante at canonical.com>
Reported-by: Venkat Rao Bagalkote <venkat88 at linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88 at linux.ibm.com>
Closes: https://lore.kernel.org/all/364996ce-aba2-4213-8d20-7dd481b43fe6@linux.ibm.com/
Signed-off-by: Madhavan Srinivasan <maddy at linux.ibm.com>
Link: https://patch.msgid.link/20260907215420.1258678-1-thibault.ferrante@canonical.com
(cherry picked from commit ed28b16eab705071d28edaace47189c2eb3aa108)
Signed-off-by: Thibault Ferrante <thibault.ferrante at canonical.com>
---
 tools/testing/selftests/powerpc/tm/tm.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/powerpc/tm/tm.h b/tools/testing/selftests/powerpc/tm/tm.h
index c03c6e7788767..6024ce4ba6ffd 100644
--- a/tools/testing/selftests/powerpc/tm/tm.h
+++ b/tools/testing/selftests/powerpc/tm/tm.h
@@ -105,8 +105,12 @@ static inline bool failure_is_nesting(void)
 static inline int tcheck(void)
 {
 	long cr;
-	asm volatile ("tcheck 0" : "=r"(cr) : : "cr0");
-	return (cr >> 28) & 4;
+	asm volatile("tcheck 0;"
+		     "mfcr %0;"
+		     : "=r"(cr)
+		     :
+		     : "cr0");
+	return (cr >> 28) & 0xf;
 }
 
 static inline bool tcheck_doomed(void)
-- 
2.55.0