signal: Deliver SIGTRAP on perf event asynchronously if blocked

With SIGTRAP on perf events, we have encountered termination of
processes due to user space attempting to block delivery of SIGTRAP.
Consider this case:

    <set up SIGTRAP on a perf event>
    ...
    sigset_t s;
    sigemptyset(&s);
    sigaddset(&s, SIGTRAP | <and others>);
    sigprocmask(SIG_BLOCK, &s, ...);
    ...
    <perf event triggers>

When the perf event triggers, while SIGTRAP is blocked, force_sig_perf()
will force the signal, but revert back to the default handler, thus
terminating the task.

This makes sense for error conditions, but not so much for explicitly
requested monitoring. However, the expectation is still that signals
generated by perf events are synchronous, which will no longer be the
case if the signal is blocked and delivered later.

To give user space the ability to clearly distinguish synchronous from
asynchronous signals, introduce siginfo_t::si_perf_flags and
TRAP_PERF_FLAG_ASYNC (opted for flags in case more binary information is
required in future).

The resolution to the problem is then to (a) no longer force the signal
(avoiding the terminations), but (b) tell user space via si_perf_flags
if the signal was synchronous or not, so that such signals can be
handled differently (e.g. let user space decide to ignore or consider
the data imprecise).

The alternative of making the kernel ignore SIGTRAP on perf events if
the signal is blocked may work for some usecases, but likely causes
issues in others that then have to revert back to interception of
sigprocmask() (which we want to avoid). [ A concrete example: when using
breakpoint perf events to track data-flow, in a region of code where
signals are blocked, data-flow can no longer be tracked accurately.
When a relevant asynchronous signal is received after unblocking the
signal, the data-flow tracking logic needs to know its state is
imprecise. ]

Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20220404111204.935357-1-elver@google.com

1 files changed, 7 insertions, 0 deletions

diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 3ba180f550d7..ffbe4cec9f32 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -99,6 +99,7 @@ union __sifields {
 			struct {
 				unsigned long _data;
 				__u32 _type;
+				__u32 _flags;
 			} _perf;
 		};
 	} _sigfault;
@@ -164,6 +165,7 @@ typedef struct siginfo {
 #define si_pkey		_sifields._sigfault._addr_pkey._pkey
 #define si_perf_data	_sifields._sigfault._perf._data
 #define si_perf_type	_sifields._sigfault._perf._type
+#define si_perf_flags	_sifields._sigfault._perf._flags
 #define si_band		_sifields._sigpoll._band
 #define si_fd		_sifields._sigpoll._fd
 #define si_call_addr	_sifields._sigsys._call_addr
@@ -271,6 +273,11 @@ typedef struct siginfo {
  */
 
 /*
+ * Flags for si_perf_flags if SIGTRAP si_code is TRAP_PERF.
+ */
+#define TRAP_PERF_FLAG_ASYNC (1u << 0)
+
+/*
  * SIGCHLD si_codes
  */
 #define CLD_EXITED	1	/* child has exited */