[PATCH v2] arm64/signal: Silence sparse warning storing GCSPR_EL0

[Mark Brown]

We are seeing a sparse warning in gcs_restore_signal():

  arch/arm64/kernel/signal.c:1054:9: sparse: sparse: cast removes address space '__user' of expression

when storing the final GCSPR_EL0 value back into the register, caused by
the fact that write_sysreg_s() casts the value it writes to a u64 which
sparse sees as discarding the __userness of the pointer.  Add a __force
cast to tell sparse that this is intentional.

While we're at it also remove spurious casts of the gcspr_el0 value as we
manipulate it which were the result of bitrot as the code was tweaked in
the long period it was out of tree.

Reported-by: kernel test robot <lkp at intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412082005.OBJ0BbWs-lkp@intel.com/
Signed-off-by: Mark Brown <broonie at kernel.org>
---
Changes in v2:
- Use __force u64 rather than unsigned long.
- Tweak commit message.
- Link to v1: https://lore.kernel.org/r/20241210-arm64-gcs-signal-sparse-v1-1-26888bcd6f89@kernel.org
---
 arch/arm64/kernel/signal.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 14ac6fdb872b9672e4b16a097f1b577aae8dec50..08d51fabdb9d47c848f14c9b25d6be04f109c2ee 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -39,7 +39,7 @@
 #ifdef CONFIG_ARM64_GCS
 #define GCS_SIGNAL_CAP(addr) (((unsigned long)addr) & GCS_CAP_ADDR_MASK)
 
-static bool gcs_signal_cap_valid(u64 addr, u64 val)
+static bool gcs_signal_cap_valid(unsigned long __user *addr, u64 val)
 {
 	return val == GCS_SIGNAL_CAP(addr);
 }
@@ -1094,15 +1094,15 @@ static int gcs_restore_signal(void)
 	/*
 	 * Check that the cap is the actual GCS before replacing it.
 	 */
-	if (!gcs_signal_cap_valid((u64)gcspr_el0, cap))
+	if (!gcs_signal_cap_valid(gcspr_el0, cap))
 		return -EINVAL;
 
 	/* Invalidate the token to prevent reuse */
-	put_user_gcs(0, (__user void*)gcspr_el0, &ret);
+	put_user_gcs(0, gcspr_el0, &ret);
 	if (ret != 0)
 		return -EFAULT;
 
-	write_sysreg_s(gcspr_el0 + 1, SYS_GCSPR_EL0);
+	write_sysreg_s((__force u64)(gcspr_el0 + 1), SYS_GCSPR_EL0);
 
 	return 0;
 }

---
base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4
change-id: 20241209-arm64-gcs-signal-sparse-53fa9cad67f7

Best regards,
-- 
Mark Brown <broonie at kernel.org>