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// SPDX-License-Identifier: GPL-2.0
/*
* Augment the raw_syscalls tracepoints with the contents of the pointer arguments.
*
* Test it with:
*
* perf trace -e tools/perf/examples/bpf/augmented_raw_syscalls.c cat /etc/passwd > /dev/null
*
* This exactly matches what is marshalled into the raw_syscall:sys_enter
* payload expected by the 'perf trace' beautifiers.
*
* For now it just uses the existing tracepoint augmentation code in 'perf
* trace', in the next csets we'll hook up these with the sys_enter/sys_exit
* code that will combine entry/exit in a strace like way.
*/
#include <stdio.h>
#include <linux/socket.h>
/* bpf-output associated map */
struct bpf_map SEC("maps") __augmented_syscalls__ = {
.type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
.key_size = sizeof(int),
.value_size = sizeof(u32),
.max_entries = __NR_CPUS__,
};
struct syscall_enter_args {
unsigned long long common_tp_fields;
long syscall_nr;
unsigned long args[6];
};
struct syscall_exit_args {
unsigned long long common_tp_fields;
long syscall_nr;
long ret;
};
struct augmented_filename {
unsigned int size;
int reserved;
char value[256];
};
#define SYS_OPEN 2
#define SYS_OPENAT 257
SEC("raw_syscalls:sys_enter")
int sys_enter(struct syscall_enter_args *args)
{
struct {
struct syscall_enter_args args;
struct augmented_filename filename;
} augmented_args;
unsigned int len = sizeof(augmented_args);
const void *filename_arg = NULL;
probe_read(&augmented_args.args, sizeof(augmented_args.args), args);
/*
* Yonghong and Edward Cree sayz:
*
* https://www.spinics.net/lists/netdev/msg531645.html
*
* >> R0=inv(id=0) R1=inv2 R6=ctx(id=0,off=0,imm=0) R7=inv64 R10=fp0,call_-1
* >> 10: (bf) r1 = r6
* >> 11: (07) r1 += 16
* >> 12: (05) goto pc+2
* >> 15: (79) r3 = *(u64 *)(r1 +0)
* >> dereference of modified ctx ptr R1 off=16 disallowed
* > Aha, we at least got a different error message this time.
* > And indeed llvm has done that optimisation, rather than the more obvious
* > 11: r3 = *(u64 *)(r1 +16)
* > because it wants to have lots of reads share a single insn. You may be able
* > to defeat that optimisation by adding compiler barriers, idk. Maybe someone
* > with llvm knowledge can figure out how to stop it (ideally, llvm would know
* > when it's generating for bpf backend and not do that). -O0? ¯\_(ツ)_/¯
*
* The optimization mostly likes below:
*
* br1:
* ...
* r1 += 16
* goto merge
* br2:
* ...
* r1 += 20
* goto merge
* merge:
* *(u64 *)(r1 + 0)
*
* The compiler tries to merge common loads. There is no easy way to
* stop this compiler optimization without turning off a lot of other
* optimizations. The easiest way is to add barriers:
*
* __asm__ __volatile__("": : :"memory")
*
* after the ctx memory access to prevent their down stream merging.
*/
switch (augmented_args.args.syscall_nr) {
case SYS_OPEN: filename_arg = (const void *)args->args[0];
__asm__ __volatile__("": : :"memory");
break;
case SYS_OPENAT: filename_arg = (const void *)args->args[1];
break;
}
if (filename_arg != NULL) {
augmented_args.filename.reserved = 0;
augmented_args.filename.size = probe_read_str(&augmented_args.filename.value,
sizeof(augmented_args.filename.value),
filename_arg);
if (augmented_args.filename.size < sizeof(augmented_args.filename.value)) {
len -= sizeof(augmented_args.filename.value) - augmented_args.filename.size;
len &= sizeof(augmented_args.filename.value) - 1;
}
} else {
len = sizeof(augmented_args.args);
}
perf_event_output(args, &__augmented_syscalls__, BPF_F_CURRENT_CPU, &augmented_args, len);
return 0;
}
SEC("raw_syscalls:sys_exit")
int sys_exit(struct syscall_exit_args *args)
{
return 1; /* 0 as soon as we start copying data returned by the kernel, e.g. 'read' */
}
license(GPL);
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