arm64/signal: Raise limit on stack frames

The signal code has a limit of 64K on the size of a stack frame that it
will generate, if this limit is exceeded then a process will be killed if
it receives a signal. Unfortunately with the advent of SME this limit is
too small - the maximum possible size of the ZA register alone is 64K. This
is not an issue for practical systems at present but is easily seen using
virtual platforms.

Raise the limit to 256K, this is substantially more than could be used by
any current architecture extension.
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220817182324.638214-2-broonie@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/signal.c

@@ -91,7 +91,7 @@ static size_t sigframe_size(struct rt_sigframe_user_layout const *user)
  * not taken into account.  This limit is not a guarantee and is
  * NOT ABI.
  */
-#define SIGFRAME_MAXSZ SZ_64K
+#define SIGFRAME_MAXSZ SZ_256K
 
 static int __sigframe_alloc(struct rt_sigframe_user_layout *user,
 			    unsigned long *offset, size_t size, bool extend)