[PATCH v5 01/23] perf trace: Set the augmented arg header in the augmenters that omit it

[Ian Rogers <irogers@google.com>]

An augmented argument is a struct augmented_arg header, holding the
length of the payload and an error code, followed by the payload. The
augmenters build it in augmented_args_tmp, a single entry per-CPU array
reused by every syscall on that CPU, so a field left unassigned holds
whatever the previous syscall on that CPU put there rather than anything
about this one.

The string augmenters get both fields from augmented_arg__read_str(),
and sys_enter_perf_event_open() bails out when its read fails, but
sys_enter_connect(), sys_enter_sendto(), sys_enter_clock_nanosleep() and
sys_enter_nanosleep() copy the payload and output the record without
ever describing it, and augment_arg() sets the length but not the error.

This has gone unnoticed because the beautifiers for those payloads take
a fixed sized type and so read the value without consulting the header:
syscall_arg__scnprintf_augmented_sockaddr(),
syscall_arg__scnprintf_augmented_timespec() and
syscall_arg__scnprintf_augmented_perf_event_attr() all cast
augmented.args->value directly. Nothing has yet read a length that was
never written.

Describe the payload everywhere one is produced, so that a reader can
bound it by its length.

These reads can fail, and none of the four checked whether they had.
Rather than claim a payload that was not read, report a length of zero
and the error, so the record describes what it holds and leaves the
scratch out of it. A reader that bounds the payload by the header then
shows the pointer, as it does for a syscall with no augmentation at
all, rather than another task's data.

The error is set for the same reason the length is, that the header is
in a buffer the next syscall on this CPU will reuse and so carries the
previous one's value if it is not written, rather than because anything
reads it yet: beauty.h still calls the field int_arg and no beautifier
looks at it. augment_arg() reported a string it could not read as an
empty one before this and still does.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 .../bpf_skel/augmented_raw_syscalls.bpf.c     | 53 ++++++++++++++++---
 1 file changed, 47 insertions(+), 6 deletions(-)

diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
index 3bc9e28a9b8a..dd3aa5bd910b 100644
--- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
+++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
@@ -210,6 +210,7 @@ int sys_enter_connect(struct syscall_enter_args *args)
 	const void *sockaddr_arg = (const void *)args->args[1];
 	unsigned int socklen = args->args[2];
 	unsigned int len = sizeof(u64) + sizeof(augmented_args->args); // the size + err in all 'augmented_arg' structs
+	int err;
 
         if (augmented_args == NULL)
                 return 1; /* Failure: don't filter */
@@ -217,9 +218,16 @@ int sys_enter_connect(struct syscall_enter_args *args)
 	_Static_assert(is_power_of_2(sizeof(augmented_args->arg.saddr)), "sizeof(augmented_args->arg.saddr) needs to be a power of two");
 	socklen &= sizeof(augmented_args->arg.saddr) - 1;
 
-	bpf_probe_read_user(&augmented_args->arg.saddr, socklen, sockaddr_arg);
+	err = bpf_probe_read_user(&augmented_args->arg.saddr, socklen, sockaddr_arg);
+	/*
+	 * A failed read leaves the scratch holding whatever the previous
+	 * syscall on this CPU put there, so say there is no payload and why,
+	 * rather than describing another task's data as this task's sockaddr.
+	 */
+	if (err < 0)
+		socklen = 0;
 	augmented_args->arg.size = socklen;
-	augmented_args->arg.err = 0;
+	augmented_args->arg.err = err < 0 ? err : 0;
 
 	return augmented__output(args, augmented_args, len + socklen);
 }
@@ -231,13 +239,19 @@ int sys_enter_sendto(struct syscall_enter_args *args)
 	const void *sockaddr_arg = (const void *)args->args[4];
 	unsigned int socklen = args->args[5];
 	unsigned int len = sizeof(u64) + sizeof(augmented_args->args); // the size + err in all 'augmented_arg' structs
+	int err;
 
         if (augmented_args == NULL)
                 return 1; /* Failure: don't filter */
 
 	socklen &= sizeof(augmented_args->arg.saddr) - 1;
 
-	bpf_probe_read_user(&augmented_args->arg.saddr, socklen, sockaddr_arg);
+	err = bpf_probe_read_user(&augmented_args->arg.saddr, socklen, sockaddr_arg);
+	/* As in sys_enter_connect(), do not describe scratch as a sockaddr. */
+	if (err < 0)
+		socklen = 0;
+	augmented_args->arg.size = socklen;
+	augmented_args->arg.err = err < 0 ? err : 0;
 
 	return augmented__output(args, augmented_args, len + socklen);
 }
@@ -372,6 +386,9 @@ int sys_enter_perf_event_open(struct syscall_enter_args *args)
 	if (bpf_probe_read_user(&augmented_args->arg.value, size, attr) < 0)
 		goto failure;
 
+	augmented_args->arg.size = size;
+	augmented_args->arg.err = 0;
+
 	return augmented__output(args, augmented_args, len + size);
 failure:
 	return 1; /* Failure: don't filter */
@@ -384,6 +401,7 @@ int sys_enter_clock_nanosleep(struct syscall_enter_args *args)
 	const void *rqtp_arg = (const void *)args->args[2];
 	unsigned int len = sizeof(u64) + sizeof(augmented_args->args); // the size + err in all 'augmented_arg' structs
 	__u32 size = sizeof(struct timespec64);
+	int err;
 
         if (augmented_args == NULL)
 		goto failure;
@@ -391,7 +409,12 @@ int sys_enter_clock_nanosleep(struct syscall_enter_args *args)
 	if (size > sizeof(augmented_args->arg.value))
                 goto failure;
 
-	bpf_probe_read_user(&augmented_args->arg.value, size, rqtp_arg);
+	err = bpf_probe_read_user(&augmented_args->arg.value, size, rqtp_arg);
+	/* As in sys_enter_connect(), do not describe scratch as a timespec. */
+	if (err < 0)
+		size = 0;
+	augmented_args->arg.size = size;
+	augmented_args->arg.err = err < 0 ? err : 0;
 
 	return augmented__output(args, augmented_args, len + size);
 failure:
@@ -405,6 +428,7 @@ int sys_enter_nanosleep(struct syscall_enter_args *args)
 	const void *req_arg = (const void *)args->args[0];
 	unsigned int len = sizeof(augmented_args->args);
 	__u32 size = sizeof(struct timespec64);
+	int err;
 
         if (augmented_args == NULL)
 		goto failure;
@@ -412,7 +436,12 @@ int sys_enter_nanosleep(struct syscall_enter_args *args)
 	if (size > sizeof(augmented_args->arg.value))
                 goto failure;
 
-	bpf_probe_read_user(&augmented_args->arg.value, size, req_arg);
+	err = bpf_probe_read_user(&augmented_args->arg.value, size, req_arg);
+	/* As in sys_enter_connect(), do not describe scratch as a timespec. */
+	if (err < 0)
+		size = 0;
+	augmented_args->arg.size = size;
+	augmented_args->arg.err = err < 0 ? err : 0;
 
 	return augmented__output(args, augmented_args, len + size);
 failure:
@@ -445,6 +474,7 @@ static inline int augment_arg(struct syscall_enter_args *args, int i,
 			      struct beauty_payload_enter *payload, u64 offset)
 {
 	int index, value_size = sizeof(struct augmented_arg) - offsetof(struct augmented_arg, value);
+	int read_err = 0;
 	struct augmented_arg *payload_offset;
 	s64 aug_size, size;
 	bool augmented;
@@ -467,8 +497,18 @@ static inline int augment_arg(struct syscall_enter_args *args, int i,
 	if (size == 1) { /* string */
 		aug_size = bpf_probe_read_user_str(payload_offset->value, value_size, arg);
 		/* minimum of 0 to pass the verifier */
-		if (aug_size < 0)
+		if (aug_size < 0) {
+			/*
+			 * Record why nothing was read. The header sits in
+			 * scratch that the next syscall on this CPU reuses,
+			 * so an error left unwritten is the previous one's.
+			 * No beautifier reads it yet, beauty.h still calls
+			 * the field int_arg, so the string is still shown
+			 * as an empty one.
+			 */
+			read_err = aug_size;
 			aug_size = 0;
+		}
 
 		augmented = true;
 	} else if (size > 0 && size <= value_size) { /* struct */
@@ -498,6 +538,7 @@ static inline int augment_arg(struct syscall_enter_args *args, int i,
 			return -1;
 
 		payload_offset->size = aug_size;
+		payload_offset->err = read_err;
 		return written;
 	}
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog