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Subject[PATCH/RFC] Make perf_event_open() propagate errors for use in bpf_perf_event_open()
Hi Peter,

bpf_perf_event_open() already returns a value, but if
perf_event_output's output_begin (mostly perf_output_begin) fails,
the only way to know about that is looking before/after the rb->lost,
right?

For ring buffer users that is ok, we'll get a PERF_RECORD_LOST,
etc, but for BPF programs it would be convenient to get that -ENOSPC and
do some fallback, whatever makes sense, like in my augmented_syscalls
stuff for 'perf trace', i.e. don't augment it (i.e. push stuff at the
end of the normal payload), just don't filter the
raw_syscalls:sys_enter, 'perf trace' will get the enter syscall enter
event without the pointer dereference at the end, etc, warn the user but
don't lose a syscall in the strace-like output.

What do you think? Am I missing something? Probably ;-)

Ah, its just test built.

- Arnaldo

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 1d5c551a5add..9ed2af2abd6d 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -978,9 +978,9 @@ extern void perf_event_output_forward(struct perf_event *event,
extern void perf_event_output_backward(struct perf_event *event,
struct perf_sample_data *data,
struct pt_regs *regs);
-extern void perf_event_output(struct perf_event *event,
- struct perf_sample_data *data,
- struct pt_regs *regs);
+extern int perf_event_output(struct perf_event *event,
+ struct perf_sample_data *data,
+ struct pt_regs *regs);

static inline bool
is_default_overflow_handler(struct perf_event *event)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3cd13a30f732..dcbb2b508034 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6489,7 +6489,7 @@ void perf_prepare_sample(struct perf_event_header *header,
data->phys_addr = perf_virt_to_phys(data->addr);
}

-static __always_inline void
+static __always_inline int
__perf_event_output(struct perf_event *event,
struct perf_sample_data *data,
struct pt_regs *regs,
@@ -6499,13 +6499,15 @@ __perf_event_output(struct perf_event *event,
{
struct perf_output_handle handle;
struct perf_event_header header;
+ int err;

/* protect the callchain buffers */
rcu_read_lock();

perf_prepare_sample(&header, data, event, regs);

- if (output_begin(&handle, event, header.size))
+ err = output_begin(&handle, event, header.size);
+ if (err)
goto exit;

perf_output_sample(&handle, &header, data, event);
@@ -6514,6 +6516,7 @@ __perf_event_output(struct perf_event *event,

exit:
rcu_read_unlock();
+ return err;
}

void
@@ -6532,12 +6535,12 @@ perf_event_output_backward(struct perf_event *event,
__perf_event_output(event, data, regs, perf_output_begin_backward);
}

-void
+int
perf_event_output(struct perf_event *event,
struct perf_sample_data *data,
struct pt_regs *regs)
{
- __perf_event_output(event, data, regs, perf_output_begin);
+ return __perf_event_output(event, data, regs, perf_output_begin);
}

/*
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 8b068adb9da1..088c2032ceaf 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -431,8 +431,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
if (unlikely(event->oncpu != cpu))
return -EOPNOTSUPP;

- perf_event_output(event, sd, regs);
- return 0;
+ return perf_event_output(event, sd, regs);
}

BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
diff --git a/tools/perf/examples/bpf/augmented_raw_syscalls.c b/tools/perf/examples/bpf/augmented_raw_syscalls.c
index 53c233370fae..9e9d4c66e53c 100644
--- a/tools/perf/examples/bpf/augmented_raw_syscalls.c
+++ b/tools/perf/examples/bpf/augmented_raw_syscalls.c
@@ -141,8 +141,8 @@ int sys_enter(struct syscall_enter_args *args)
len = sizeof(augmented_args.args);
}

- perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, &augmented_args, len);
- return 0;
+ /* If perf_event_output fails, return non-zero so that it gets recorded unaugmented */
+ return perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, &augmented_args, len);
}

SEC("raw_syscalls:sys_exit")
diff --git a/tools/perf/examples/bpf/augmented_syscalls.c b/tools/perf/examples/bpf/augmented_syscalls.c
index 2ae44813ef2d..b7dba114e36c 100644
--- a/tools/perf/examples/bpf/augmented_syscalls.c
+++ b/tools/perf/examples/bpf/augmented_syscalls.c
@@ -55,9 +55,9 @@ int syscall_enter(syscall)(struct syscall_enter_##syscall##_args *args) \
len -= sizeof(augmented_args.filename.value) - augmented_args.filename.size; \
len &= sizeof(augmented_args.filename.value) - 1; \
} \
- perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
- &augmented_args, len); \
- return 0; \
+ /* If perf_event_output fails, return non-zero so that it gets recorded unaugmented */ \
+ return perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
+ &augmented_args, len); \
} \
int syscall_exit(syscall)(struct syscall_exit_args *args) \
{ \
@@ -125,10 +125,10 @@ int syscall_enter(syscall)(struct syscall_enter_##syscall##_args *args) \
/* addrlen = augmented_args.args.addrlen; */ \
/* */ \
probe_read(&augmented_args.addr, addrlen, args->addr_ptr); \
- perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
- &augmented_args, \
- sizeof(augmented_args) - sizeof(augmented_args.addr) + addrlen); \
- return 0; \
+ /* If perf_event_output fails, return non-zero so that it gets recorded unaugmented */ \
+ return perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
+ &augmented_args, \
+ sizeof(augmented_args) - sizeof(augmented_args.addr) + addrlen);\
} \
int syscall_exit(syscall)(struct syscall_exit_args *args) \
{ \
diff --git a/tools/perf/examples/bpf/etcsnoop.c b/tools/perf/examples/bpf/etcsnoop.c
index b59e8812ee8c..550e69c2e8d1 100644
--- a/tools/perf/examples/bpf/etcsnoop.c
+++ b/tools/perf/examples/bpf/etcsnoop.c
@@ -49,11 +49,11 @@ int syscall_enter(syscall)(struct syscall_enter_##syscall##_args *args) \
args->filename_ptr); \
if (__builtin_memcmp(augmented_args.filename.value, etc, 4) != 0) \
return 0; \
- perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
- &augmented_args, \
- (sizeof(augmented_args) - sizeof(augmented_args.filename.value) + \
- augmented_args.filename.size)); \
- return 0; \
+ /* If perf_event_output fails, return non-zero so that it gets recorded unaugmented */ \
+ return perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, \
+ &augmented_args, \
+ (sizeof(augmented_args) - sizeof(augmented_args.filename.value) + \
+ augmented_args.filename.size)); \
}

struct syscall_enter_openat_args {
\
 
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