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samples/bpf: Fix tracex2 by using BPF_KSYSCALL macro

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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>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221224071527.2292-5-danieltimlee@gmail.com
This commit is contained in:
Daniel T. Lee 2022-12-24 16:15:25 +09:00 committed by Andrii Nakryiko
parent d4fffba4d0
commit 2e5c4dd7f8
1 changed files with 3 additions and 3 deletions
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6
samples/bpf/tracex2.bpf.c
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@ -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));