Subject: [PATCH 2/2] powerpc/signal: Report minimum signal frame size to userspace via AT_MINSIGSTKSZ

From: Nicholas Piggin <[email-masked]>
Date: Tue, 8 Mar 2022 04:27:34 +1000

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.

Cc: Alan Modra <[email-masked]>
Cc: Tulio Magno Quites Machado Filho <[email-masked]>
References: 94b07c1f8c39 ("arm64: signal: Report signal frame size to userspace via auxv")
Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/include/asm/elf.h         | 14 +++++++++++++-
 arch/powerpc/include/asm/signal.h      |  5 +++++
 arch/powerpc/include/uapi/asm/auxvec.h |  4 +++-
 arch/powerpc/kernel/signal.c           | 15 +++++++++++++++
 arch/powerpc/kernel/signal_32.c        |  6 ++++++
 arch/powerpc/kernel/signal_64.c        |  5 +++++
 6 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/elf.h b/arch/powerpc/include/asm/elf.h
index b8425e3cfd81..19d5c798c566 100644
--- a/arch/powerpc/include/asm/elf.h
+++ b/arch/powerpc/include/asm/elf.h
@@ -160,7 +160,7 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
  *   even if DLINFO_ARCH_ITEMS goes to zero or is undefined.
  * update AT_VECTOR_SIZE_ARCH if the number of NEW_AUX_ENT entries changes
  */
-#define ARCH_DLINFO							\
+#define COMMON_ARCH_DLINFO						\
 do {									\
 	/* Handle glibc compatibility. */				\
 	NEW_AUX_ENT(AT_IGNOREPPC, AT_IGNOREPPC);			\
@@ -173,6 +173,18 @@ do {									\
 	ARCH_DLINFO_CACHE_GEOMETRY;					\
 } while (0)
 
+#define ARCH_DLINFO							\
+do {									\
+	COMMON_ARCH_DLINFO;						\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_min_sigframe_size());		\
+} while (0)
+
+#define COMPAT_ARCH_DLINFO						\
+do {									\
+	COMMON_ARCH_DLINFO;						\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_min_sigframe_size_compat());	\
+} while (0)
+
 /* Relocate the kernel image to @final_address */
 void relocate(unsigned long final_address);
 
diff --git a/arch/powerpc/include/asm/signal.h b/arch/powerpc/include/asm/signal.h
index 99e1c6de27bc..922d43700fb4 100644
--- a/arch/powerpc/include/asm/signal.h
+++ b/arch/powerpc/include/asm/signal.h
@@ -9,4 +9,9 @@
 struct pt_regs;
 void do_notify_resume(struct pt_regs *regs, unsigned long thread_info_flags);
 
+unsigned long get_min_sigframe_size_32(void);
+unsigned long get_min_sigframe_size_64(void);
+unsigned long get_min_sigframe_size(void);
+unsigned long get_min_sigframe_size_compat(void);
+
 #endif /* _ASM_POWERPC_SIGNAL_H */
diff --git a/arch/powerpc/include/uapi/asm/auxvec.h b/arch/powerpc/include/uapi/asm/auxvec.h
index 7af21dc0e320..aa7c16215453 100644
--- a/arch/powerpc/include/uapi/asm/auxvec.h
+++ b/arch/powerpc/include/uapi/asm/auxvec.h
@@ -48,6 +48,8 @@
 #define AT_L3_CACHESIZE		46
 #define AT_L3_CACHEGEOMETRY	47
 
-#define AT_VECTOR_SIZE_ARCH	14 /* entries in ARCH_DLINFO */
+#define AT_MINSIGSTKSZ		51      /* stack needed for signal delivery */
+
+#define AT_VECTOR_SIZE_ARCH	15 /* entries in ARCH_DLINFO */
 
 #endif
diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index b93b87df499d..b3c458db2b8d 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 d1e1fc0acbea..7c5317e07169 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,
-- 
2.23.0