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authorPeter Collingbourne <pcc@google.com>2020-11-12 18:53:34 -0800
committerEric W. Biederman <ebiederm@xmission.com>2020-11-23 10:31:05 -0600
commit23acdc76f1798b090bb9dcc90671cd29d929834e (patch)
tree32d2940740e2ca405377686c6e13e3710a03e931 /kernel/signal.c
parent1d82b7898f2ad9cc414805aef23b99b742218f10 (diff)
signal: clear non-uapi flag bits when passing/returning sa_flags
Previously we were not clearing non-uapi flag bits in sigaction.sa_flags when storing the userspace-provided sa_flags or when returning them via oldact. Start doing so. This allows userspace to detect missing support for flag bits and allows the kernel to use non-uapi bits internally, as we are already doing in arch/x86 for two flag bits. Now that this change is in place, we no longer need the code in arch/x86 that was hiding these bits from userspace, so remove it. This is technically a userspace-visible behavior change for sigaction, as the unknown bits returned via oldact.sa_flags are no longer set. However, we are free to define the behavior for unknown bits exactly because their behavior is currently undefined, so for now we can define the meaning of each of them to be "clear the bit in oldact.sa_flags unless the bit becomes known in the future". Furthermore, this behavior is consistent with OpenBSD [1], illumos [2] and XNU [3] (FreeBSD [4] and NetBSD [5] fail the syscall if unknown bits are set). So there is some precedent for this behavior in other kernels, and in particular in XNU, which is probably the most popular kernel among those that I looked at, which means that this change is less likely to be a compatibility issue. Link: [1] https://github.com/openbsd/src/blob/f634a6a4b5bf832e9c1de77f7894ae2625e74484/sys/kern/kern_sig.c#L278 Link: [2] https://github.com/illumos/illumos-gate/blob/76f19f5fdc974fe5be5c82a556e43a4df93f1de1/usr/src/uts/common/syscall/sigaction.c#L86 Link: [3] https://github.com/apple/darwin-xnu/blob/a449c6a3b8014d9406c2ddbdc81795da24aa7443/bsd/kern/kern_sig.c#L480 Link: [4] https://github.com/freebsd/freebsd/blob/eded70c37057857c6e23fae51f86b8f8f43cd2d0/sys/kern/kern_sig.c#L699 Link: [5] https://github.com/NetBSD/src/blob/3365779becdcedfca206091a645a0e8e22b2946e/sys/kern/sys_sig.c#L473 Signed-off-by: Peter Collingbourne <pcc@google.com> Reviewed-by: Dave Martin <Dave.Martin@arm.com> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com> Link: https://linux-review.googlesource.com/id/I35aab6f5be932505d90f3b3450c083b4db1eca86 Link: https://lkml.kernel.org/r/878dbcb5f47bc9b11881c81f745c0bef5c23f97f.1605235762.git.pcc@google.com Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Diffstat (limited to 'kernel/signal.c')
-rw-r--r--kernel/signal.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/kernel/signal.c b/kernel/signal.c
index ef8f2a28d37c..8f5bd12ee41b 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3985,6 +3985,16 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
if (oact)
*oact = *k;
+ /*
+ * 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.
+ */
+ if (act)
+ act->sa.sa_flags &= UAPI_SA_FLAGS;
+ if (oact)
+ oact->sa.sa_flags &= UAPI_SA_FLAGS;
+
sigaction_compat_abi(act, oact);
if (act) {

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