[linux.git] /

// 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 <unistd.h>
#include <pid_filter.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 {
        bool    enabled;
};

struct bpf_map SEC("maps") syscalls = {
        .type        = BPF_MAP_TYPE_ARRAY,
        .key_size    = sizeof(int),
        .value_size  = sizeof(struct syscall),
        .max_entries = 512,
};

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_ACCESS 21
#define SYS_OPENAT 257

pid_filter(pids_filtered);

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;
        struct syscall *syscall;
        unsigned int len = sizeof(augmented_args);
        const void *filename_arg = NULL;

        if (pid_filter__has(&pids_filtered, getpid()))
                return 0;

        probe_read(&augmented_args.args, sizeof(augmented_args.args), args);

        syscall = bpf_map_lookup_elem(&syscalls, &augmented_args.args.syscall_nr);
        if (syscall == NULL || !syscall->enabled)
                return 0;
        /*
         * 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_ACCESS:
        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)
{
        struct syscall_exit_args exit_args;
        struct syscall *syscall;

        if (pid_filter__has(&pids_filtered, getpid()))
                return 0;

        probe_read(&exit_args, sizeof(exit_args), args);

        syscall = bpf_map_lookup_elem(&syscalls, &exit_args.syscall_nr);
        if (syscall == NULL || !syscall->enabled)
                return 0;

        return 1;
}

license(GPL);