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authorPeter Collingbourne <pcc@google.com>2020-11-16 19:17:25 -0800
committerEric W. Biederman <ebiederm@xmission.com>2020-11-23 10:31:06 -0600
commita54f0dfda754c5cecc89a14dab68a3edc1e497b5 (patch)
treea8899e5a502ef6a262acbbf073d155c6c87c892d /kernel/signal.c
parent7da5082a2f9a1d16eded00c204fdb52a855c2bb2 (diff)
signal: define the SA_UNSUPPORTED bit in sa_flags
Define a sa_flags bit, SA_UNSUPPORTED, which will never be supported in the uapi. The purpose of this flag bit is to allow userspace to distinguish an old kernel that does not clear unknown sa_flags bits from a kernel that supports every flag bit. In other words, if userspace does something like: act.sa_flags |= SA_UNSUPPORTED; sigaction(SIGSEGV, &act, 0); sigaction(SIGSEGV, 0, &oldact); and finds that SA_UNSUPPORTED remains set in oldact.sa_flags, it means that the kernel cannot be trusted to have cleared unknown flag bits from sa_flags, so no assumptions about flag bit support can be made. Signed-off-by: Peter Collingbourne <pcc@google.com> Reviewed-by: Dave Martin <Dave.Martin@arm.com> Link: https://linux-review.googlesource.com/id/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb Link: https://lkml.kernel.org/r/bda7ddff8895a9bc4ffc5f3cf3d4d37a32118077.1605582887.git.pcc@google.com Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Diffstat (limited to 'kernel/signal.c')
-rw-r--r--kernel/signal.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/kernel/signal.c b/kernel/signal.c
index 8f5bd12ee41b..8f34819e80de 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3986,6 +3986,12 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
*oact = *k;
/*
+ * Make sure that we never accidentally claim to support SA_UNSUPPORTED,
+ * e.g. by having an architecture use the bit in their uapi.
+ */
+ BUILD_BUG_ON(UAPI_SA_FLAGS & SA_UNSUPPORTED);
+
+ /*
* Clear unknown flag bits in order to allow userspace to detect missing
* support for flag bits and to allow the kernel to use non-uapi bits
* internally.

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