fix(driver/modern_bpf): avoid bpf_loop() helper.

We can't use bpf_loop() helper since the `bpf_core_enum_value_exists` check
triggers a verifier failure on kernels prior to 5.13 that hadn't got `PTR_TO_FUNC` support.
See https://lore.kernel.org/bpf/CAGQdkDt9zyQwr5JyftXqL=OLKscNcqUtEteY4hvOkx2S4GdEkQ@mail.gmail.com/T/#u.

Instead, loop up to 16 messages.

Signed-off-by: Federico Di Pierro <[email protected]>

Co-authored-by: Andrea Terzolo <[email protected]>

Author: FedeDP

Diff for: driver/modern_bpf/helpers/base/shared_size.h

@@ -26,6 +26,9 @@
 /* Maximum number of `iovec` structures that we can analyze. */
 #define MAX_IOVCNT 32
 
+/* Maximum number of supported sendmmsg/recvmmsg messages */
+#define MAX_SENDMMSG_RECVMMSG_SIZE 16
+
 /* Maximum number of `pollfd` structures that we can analyze. */
 #define MAX_POLLFD 16
 

Diff for: driver/modern_bpf/programs/tail_called/events/syscall_dispatched_events/recvmmsg.bpf.c

@@ -42,7 +42,7 @@ typedef struct {
 	void *ctx;
 } recvmmsg_data_t;
 
-static long handle_exit(uint32_t index, void *ctx) {
+static __always_inline long handle_exit(uint32_t index, void *ctx) {
 	recvmmsg_data_t *data = (recvmmsg_data_t *)ctx;
 	struct mmsghdr mmh = {0};
 	if(bpf_probe_read_user((void *)&mmh,
@@ -159,14 +159,17 @@ int BPF_PROG(recvmmsg_x, struct pt_regs *regs, long ret) {
 	        .ctx = ctx,
 	};
 
-	if(bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_loop)) {
-		uint32_t nr_loops = ret < 1024 ? ret : 1024;
-		bpf_loop(nr_loops, handle_exit, &data, 0);
-	} else {
-		for(int i = 0; i < ret && i < MAX_IOVCNT; i++) {
-			handle_exit(i, &data);
-		}
+	// We can't use bpf_loop() helper since the below check triggers a verifier failure:
+	// see
+	// https://lore.kernel.org/bpf/CAGQdkDt9zyQwr5JyftXqL=OLKscNcqUtEteY4hvOkx2S4GdEkQ@mail.gmail.com/T/#u
+	/*if(bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_loop)) {
+	    uint32_t nr_loops = ret < 1024 ? ret : 1024;
+	    bpf_loop(nr_loops, handle_exit, &data, 0);
+	} else {*/
+	for(int i = 0; i < ret && i < MAX_SENDMMSG_RECVMMSG_SIZE; i++) {
+		handle_exit(i, &data);
 	}
+	//}
 
 	return 0;
 }

Diff for: driver/modern_bpf/programs/tail_called/events/syscall_dispatched_events/sendmmsg.bpf.c

@@ -42,7 +42,7 @@ typedef struct {
 	void *ctx;
 } sendmmsg_exit_t;
 
-static long handle_exit(uint32_t index, void *ctx) {
+static __always_inline long handle_exit(uint32_t index, void *ctx) {
 	sendmmsg_exit_t *data = (sendmmsg_exit_t *)ctx;
 	struct mmsghdr mmh = {0};
 
@@ -152,14 +152,17 @@ int BPF_PROG(sendmmsg_x, struct pt_regs *regs, long ret) {
 	        .ctx = ctx,
 	};
 
-	if(bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_loop)) {
-		uint32_t nr_loops = ret < 1024 ? ret : 1024;
-		bpf_loop(nr_loops, handle_exit, &data, 0);
-	} else {
-		for(int i = 0; i < ret && i < MAX_IOVCNT; i++) {
-			handle_exit(i, &data);
-		}
+	// We can't use bpf_loop() helper since the below check triggers a verifier failure:
+	// see
+	// https://lore.kernel.org/bpf/CAGQdkDt9zyQwr5JyftXqL=OLKscNcqUtEteY4hvOkx2S4GdEkQ@mail.gmail.com/T/#u
+	/*if(bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_loop)) {
+	    uint32_t nr_loops = ret < 1024 ? ret : 1024;
+	    bpf_loop(nr_loops, handle_exit, &data, 0);
+	} else {*/
+	for(int i = 0; i < ret && i < MAX_SENDMMSG_RECVMMSG_SIZE; i++) {
+		handle_exit(i, &data);
 	}
+	//}
 
 	return 0;
 }