[PATCH v6 1/3] signal: Allow architectures to store arch-specific data in kernel_siginfo

From: Peter Collingbourne <pcc@google.com>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Kostya Serebryany <kcc@google.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	 Dave Martin <Dave.Martin@arm.com>, Will Deacon <will@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	 "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrey Konovalov <andreyknvl@google.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Peter Collingbourne <pcc@google.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Richard Henderson <rth@twiddle.net>
Subject: [PATCH v6 1/3] signal: Allow architectures to store arch-specific data in kernel_siginfo
Date: Wed, 20 May 2020 19:29:41 -0700	[thread overview]
Message-ID: <20200521022943.195898-2-pcc@google.com> (raw)
In-Reply-To: <20200521022943.195898-1-pcc@google.com>

In some cases we would like to store architecture-specific data in
the kernel's siginfo but not in the userspace one. This is generally
data is conceptually part of siginfo but is not stored there for one
reason or another. For example, on arm64, the arch-specific fault
code register ESR_EL1 is exposed to signal handlers, but since it is
associated with many different types of signals it does not fit well
into siginfo and appears in sigcontext instead.

Currently this data is stored in thread_struct, which is error-prone
because the data is associated with the signal itself and not the task,
and as a result it could get out of sync with the signal that is
currently being delivered.

To help avoid these types of errors, introduce a way for architectures
to store architecture-specific data in the kernel_siginfo. This part
of the kernel_siginfo is not exposed to userspace so the architecture
can use it in any way that it likes without ABI concerns. A follow-up
change will start using this mechanism on arm64 to store the fault
code and fault address.

Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 include/linux/signal.h       |  2 --
 include/linux/signal_types.h |  3 +++
 kernel/signal.c              | 17 +++++++++++++----
 3 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/include/linux/signal.h b/include/linux/signal.h
index 05bacd2ab135..34bf18932e9b 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -22,8 +22,6 @@ static inline void clear_siginfo(kernel_siginfo_t *info)
 	memset(info, 0, sizeof(*info));
 }
 
-#define SI_EXPANSION_SIZE (sizeof(struct siginfo) - sizeof(struct kernel_siginfo))
-
 int copy_siginfo_to_user(siginfo_t __user *to, const kernel_siginfo_t *from);
 int copy_siginfo_from_user(kernel_siginfo_t *to, const siginfo_t __user *from);
 
diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
index f8a90ae9c6ec..a8635eeb890b 100644
--- a/include/linux/signal_types.h
+++ b/include/linux/signal_types.h
@@ -11,6 +11,9 @@
 
 typedef struct kernel_siginfo {
 	__SIGINFO;
+#ifdef __ARCH_HAS_PRIVATE_SIGINFO
+	struct arch_private_siginfo arch;
+#endif
 } kernel_siginfo_t;
 
 /*
diff --git a/kernel/signal.c b/kernel/signal.c
index 284fc1600063..a33df2280ed5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3180,15 +3180,21 @@ enum siginfo_layout siginfo_layout(unsigned sig, int si_code)
 	return layout;
 }
 
+struct shared_siginfo {
+	__SIGINFO;
+};
+
+#define SI_EXPANSION_SIZE (sizeof(struct siginfo) - sizeof(struct shared_siginfo))
+
 static inline char __user *si_expansion(const siginfo_t __user *info)
 {
-	return ((char __user *)info) + sizeof(struct kernel_siginfo);
+	return ((char __user *)info) + sizeof(struct shared_siginfo);
 }
 
 int copy_siginfo_to_user(siginfo_t __user *to, const kernel_siginfo_t *from)
 {
 	char __user *expansion = si_expansion(to);
-	if (copy_to_user(to, from , sizeof(struct kernel_siginfo)))
+	if (copy_to_user(to, from , sizeof(struct shared_siginfo)))
 		return -EFAULT;
 	if (clear_user(expansion, SI_EXPANSION_SIZE))
 		return -EFAULT;
@@ -3198,6 +3204,9 @@ int copy_siginfo_to_user(siginfo_t __user *to, const kernel_siginfo_t *from)
 static int post_copy_siginfo_from_user(kernel_siginfo_t *info,
 				       const siginfo_t __user *from)
 {
+#ifdef __ARCH_HAS_PRIVATE_SIGINFO
+	memset(&info->arch, 0, sizeof(info->arch));
+#endif
 	if (unlikely(!known_siginfo_layout(info->si_signo, info->si_code))) {
 		char __user *expansion = si_expansion(from);
 		char buf[SI_EXPANSION_SIZE];
@@ -3221,7 +3230,7 @@ static int post_copy_siginfo_from_user(kernel_siginfo_t *info,
 static int __copy_siginfo_from_user(int signo, kernel_siginfo_t *to,
 				    const siginfo_t __user *from)
 {
-	if (copy_from_user(to, from, sizeof(struct kernel_siginfo)))
+	if (copy_from_user(to, from, sizeof(struct shared_siginfo)))
 		return -EFAULT;
 	to->si_signo = signo;
 	return post_copy_siginfo_from_user(to, from);
@@ -3229,7 +3238,7 @@ static int __copy_siginfo_from_user(int signo, kernel_siginfo_t *to,
 
 int copy_siginfo_from_user(kernel_siginfo_t *to, const siginfo_t __user *from)
 {
-	if (copy_from_user(to, from, sizeof(struct kernel_siginfo)))
+	if (copy_from_user(to, from, sizeof(struct shared_siginfo)))
 		return -EFAULT;
 	return post_copy_siginfo_from_user(to, from);
 }
-- 
2.26.2.761.g0e0b3e54be-goog


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