[PATCH v2 2/5] selftests/mm: Fix resv_sz when parsing arm64 signal frame

[Kevin Brodsky <kevin.brodsky@arm.com>]

get_header() wants the size of the reserved area in struct
sigcontext, but instead we pass it the size of the entire struct.
This could in theory result in an out-of-bounds read (if the signal
frame is malformed).

Fix this using one of the existing macros from
tools/testing/selftests/arm64/signal/testcases/testcases.h.

This issue was reported by Sashiko on a patch that copied this
portion of the code.

Link: https://sashiko.dev/#/patchset/20260421144252.1440365-1-kevin.brodsky%40arm.com
Fixes: f5b5ea51f78f ("selftests: mm: make protection_keys test work on arm64")
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 tools/testing/selftests/mm/pkey-arm64.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/pkey-arm64.h b/tools/testing/selftests/mm/pkey-arm64.h
index 8e9685e03c44..c5a78a2f211d 100644
--- a/tools/testing/selftests/mm/pkey-arm64.h
+++ b/tools/testing/selftests/mm/pkey-arm64.h
@@ -130,9 +130,10 @@ static inline u64 get_pkey_bits(u64 reg, int pkey)
 static inline void aarch64_write_signal_pkey(ucontext_t *uctxt, u64 pkey)
 {
 	struct _aarch64_ctx *ctx = GET_UC_RESV_HEAD(uctxt);
+	size_t resv_size = GET_UCP_RESV_SIZE(uctxt);
 	struct poe_context *poe_ctx =
 		(struct poe_context *) get_header(ctx, POE_MAGIC,
-						sizeof(uctxt->uc_mcontext), NULL);
+						  resv_size, NULL);
 	if (poe_ctx)
 		poe_ctx->por_el0 = pkey;
 }

-- 
2.51.2