[RFC PATCH v4 13/13] riscv: signal: Report signal frame size to userspace via auxv

[Greentime Hu]

From: Vincent Chen <vincent.chen@xxxxxxxxxx>

The vector register belongs to the signal context. They need to be stored
and restored as entering and leaving the signal handler. According to the
V-extension specification, the maximum length of the vector registers can
be 2^(XLEN-1). Hence, if userspace refers to the MINSIGSTKSZ #define
(2KB) to create a sigframe, it may not be enough. To resolve this problem,
this patch refers to the commit 94b07c1f8c39c
("arm64: signal: Report signal frame size to userspace via auxv") to enable
userspace to know the minimum required sigframe size through the auxiliary
vector and use it to allocate enough memory for signal context.

Signed-off-by: Vincent Chen <vincent.chen@xxxxxxxxxx>
---
 arch/riscv/include/asm/elf.h         | 17 +++++++++++++----
 arch/riscv/include/asm/processor.h   |  2 ++
 arch/riscv/include/uapi/asm/auxvec.h |  2 ++
 arch/riscv/kernel/setup.c            |  5 +++++
 arch/riscv/kernel/signal.c           | 16 ++++++++++++++++
 5 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
index d83a4efd052b..b6b15fc5f784 100644
--- a/arch/riscv/include/asm/elf.h
+++ b/arch/riscv/include/asm/elf.h
@@ -57,10 +57,19 @@ extern unsigned long elf_hwcap;
 #define ELF_PLATFORM	(NULL)
 
 #ifdef CONFIG_MMU
-#define ARCH_DLINFO						\
-do {								\
-	NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
-		(elf_addr_t)current->mm->context.vdso);		\
+#define ARCH_DLINFO						 \
+do {								 \
+	NEW_AUX_ENT(AT_SYSINFO_EHDR,				 \
+		(elf_addr_t)current->mm->context.vdso);		 \
+	/*							 \
+	 * Should always be nonzero unless there's a kernel bug. \
+	 * If we haven't determined a sensible value to give to	 \
+	 * userspace, omit the entry:				 \
+	 */							 \
+	if (likely(signal_minsigstksz))				 \
+		NEW_AUX_ENT(AT_MINSIGSTKSZ, signal_minsigstksz); \
+	else							 \
+		NEW_AUX_ENT(AT_IGNORE, 0);			 \
 } while (0)
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
 struct linux_binprm;
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 217273375cfb..5be2da702897 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -7,6 +7,7 @@
 #define _ASM_RISCV_PROCESSOR_H
 
 #include <linux/const.h>
+#include <linux/cache.h>
 
 #include <asm/ptrace.h>
 
@@ -79,6 +80,7 @@ int riscv_of_processor_hartid(struct device_node *node);
 
 extern void riscv_fill_hwcap(void);
 
+extern unsigned long signal_minsigstksz __ro_after_init;
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/include/uapi/asm/auxvec.h b/arch/riscv/include/uapi/asm/auxvec.h
index d86cb17bbabe..9745a01e5e61 100644
--- a/arch/riscv/include/uapi/asm/auxvec.h
+++ b/arch/riscv/include/uapi/asm/auxvec.h
@@ -10,4 +10,6 @@
 /* vDSO location */
 #define AT_SYSINFO_EHDR 33
 
+#define AT_MINSIGSTKSZ 51
+
 #endif /* _UAPI_ASM_RISCV_AUXVEC_H */
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index 145128a7e560..6220e25ea9b0 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -17,6 +17,7 @@
 #include <linux/sched/task.h>
 #include <linux/swiotlb.h>
 #include <linux/smp.h>
+#include <linux/processor.h>
 
 #include <asm/clint.h>
 #include <asm/cpu_ops.h>
@@ -62,6 +63,8 @@ void __init parse_dtb(void)
 #endif
 }
 
+extern void __init minsigstksz_setup(void);
+
 void __init setup_arch(char **cmdline_p)
 {
 	init_mm.start_code = (unsigned long) _stext;
@@ -95,6 +98,8 @@ void __init setup_arch(char **cmdline_p)
 #endif
 
 	riscv_fill_hwcap();
+
+	minsigstksz_setup();
 }
 
 static int __init topology_init(void)
diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
index 9ada6f74bb95..4f81251867e6 100644
--- a/arch/riscv/kernel/signal.c
+++ b/arch/riscv/kernel/signal.c
@@ -404,3 +404,19 @@ asmlinkage __visible void do_notify_resume(struct pt_regs *regs,
 		tracehook_notify_resume(regs);
 	}
 }
+
+unsigned long __ro_after_init signal_minsigstksz;
+
+/*
+ * Determine the stack space required for guaranteed signal devliery.
+ * This function is used to populate AT_MINSIGSTKSZ at process startup.
+ * cpufeatures setup is assumed to be complete.
+ */
+void __init minsigstksz_setup(void)
+{
+	signal_minsigstksz = sizeof(struct rt_sigframe);
+#ifdef CONFIG_VECTOR
+	if (has_vector)
+		signal_minsigstksz += riscv_vsize;
+#endif
+}
-- 
2.26.2