powerpc/signal: Report minimum signal frame size to userspace via AT_MINSIGSTKSZ

Implement the AT_MINSIGSTKSZ AUXV entry, allowing userspace to
dynamically size stack allocations in a manner forward-compatible with
new processor state saved in the signal frame

For now these statically find the maximum signal frame size rather than
doing any runtime testing of features to minimise the size.

glibc 2.34 will take advantage of this, as will applications that use
use _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
References: 94b07c1f8c39 ("arm64: signal: Report signal frame size to userspace via auxv")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220307182734.289289-2-npiggin@gmail.com

3 files changed, 26 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index f7f8620663c7..68a91e553e14 100644
--- a/arch/powerpc/kernel/signal.c
+++ b/arch/powerpc/kernel/signal.c
@@ -141,6 +141,21 @@ unsigned long copy_ckvsx_from_user(struct task_struct *task,
 
 int show_unhandled_signals = 1;
 
+unsigned long get_min_sigframe_size(void)
+{
+	if (IS_ENABLED(CONFIG_PPC64))
+		return get_min_sigframe_size_64();
+	else
+		return get_min_sigframe_size_32();
+}
+
+#ifdef CONFIG_COMPAT
+unsigned long get_min_sigframe_size_compat(void)
+{
+	return get_min_sigframe_size_32();
+}
+#endif
+
 /*
  * Allocate space for the signal frame
  */
diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index d84c434b2b78..157a7403e3eb 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -233,6 +233,12 @@ struct rt_sigframe {
 	int			abigap[56];
 };
 
+unsigned long get_min_sigframe_size_32(void)
+{
+	return max(sizeof(struct rt_sigframe) + __SIGNAL_FRAMESIZE + 16,
+		   sizeof(struct sigframe) + __SIGNAL_FRAMESIZE);
+}
+
 /*
  * Save the current user registers on the user stack.
  * We only save the altivec/spe registers if the process has used
diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 858fc13b8c51..472596a109e2 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -66,6 +66,11 @@ struct rt_sigframe {
 	char abigap[USER_REDZONE_SIZE];
 } __attribute__ ((aligned (16)));
 
+unsigned long get_min_sigframe_size_64(void)
+{
+	return sizeof(struct rt_sigframe) + __SIGNAL_FRAMESIZE;
+}
+
 /*
  * This computes a quad word aligned pointer inside the vmx_reserve array
  * element. For historical reasons sigcontext might not be quad word aligned,