[bpf-next v3 4/6] samples/bpf: fix tracex2 by using BPF_KSYSCALL macro

Daniel T. Lee
2022-12-24T07:16:00Z

Currently, there is a problem with tracex2, as it doesn't print the
histogram properly and the results are misleading. (all results report
as 0)

The problem is caused by a change in arguments of the function to which
the kprobe connects. This tracex2 bpf program uses kprobe (attached
to __x64_sys_write) to figure out the size of the write system call. In
order to achieve this, the third argument 'count' must be intact.

The following is a prototype of the sys_write variant. (checked with
pfunct)

~/git/linux$ pfunct -P fs/read_write.o | grep sys_write
ssize_t ksys_write(unsigned int fd, const char * buf, size_t count);
long int __x64_sys_write(const struct pt_regs * regs);
... cross compile with s390x ...
long int __s390_sys_write(struct pt_regs * regs);

Since the nature of SYSCALL_WRAPPER function wraps the argument once,
additional process of argument extraction is required to properly parse
the argument.

#define BPF_KSYSCALL(name, args...)
... snip ...
struct pt_regs *regs = LINUX_HAS_SYSCALL_WRAPPER \
? (struct pt_regs *)PT_REGS_PARM1(ctx) \
: ctx; \

In order to fix this problem, the BPF_SYSCALL macro has been used. This
reduces the hassle of parsing arguments from pt_regs. Since the macro
uses the CORE version of argument extraction, additional portability
comes too.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
---
samples/bpf/tracex2.bpf.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/samples/bpf/tracex2.bpf.c b/samples/bpf/tracex2.bpf.c
index 4b9d956a3e2c..1e1a75850307 100644
--- a/samples/bpf/tracex2.bpf.c
+++ b/samples/bpf/tracex2.bpf.c
@@ -8,6 +8,7 @@
#include <linux/version.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>

struct {
__uint(type, BPF_MAP_TYPE_HASH);
@@ -76,14 +77,13 @@ struct {
} my_hist_map SEC(".maps");

SEC("ksyscall/write")
-int bpf_prog3(struct pt_regs *ctx)
+int BPF_KSYSCALL(bpf_prog3, unsigned int fd, const char *buf, size_t count)
{
- long write_size = PT_REGS_PARM3(ctx);
long init_val = 1;
long *value;
struct hist_key key;

- key.index = log2l(write_size);
+ key.index = log2l(count);
key.pid_tgid = bpf_get_current_pid_tgid();
key.uid_gid = bpf_get_current_uid_gid();
bpf_get_current_comm(&key.comm, sizeof(key.comm));
--
2.34.1