[PATCH v6 tip 4/8] tracing: allow BPF programs to call bpf_trace_printk()

Alexei Starovoitov
2015-03-11T04:20:57Z

Debugging of BPF programs needs some form of printk from the program,
so let programs call limited trace_printk() with %d %u %x %p modifiers only.

Similar to kernel modules, during program load verifier checks whether program
is calling bpf_trace_printk() and if so, kernel allocates trace_printk buffers
and emits big 'this is debug only' banner.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
---
include/uapi/linux/bpf.h | 1 +
kernel/trace/bpf_trace.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 69 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 101e509d1001..4095f3d9a716 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -166,6 +166,7 @@ enum bpf_func_id {
BPF_FUNC_map_delete_elem, /* int map_delete_elem(&map, &key) */
BPF_FUNC_probe_read, /* int bpf_probe_read(void *dst, int size, void *src) */
BPF_FUNC_ktime_get_ns, /* u64 bpf_ktime_get_ns(void) */
+ BPF_FUNC_trace_printk, /* int bpf_trace_printk(const char *fmt, int fmt_size, ...) */
__BPF_FUNC_MAX_ID,
};

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index ee7c2c629e75..367235d8576b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -62,6 +62,60 @@ static u64 bpf_ktime_get_ns(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
return ktime_get_mono_fast_ns();
}

+/* limited trace_printk()
+ * only %d %u %x %ld %lu %lx %lld %llu %llx %p conversion specifiers allowed
+ */
+static u64 bpf_trace_printk(u64 r1, u64 fmt_size, u64 r3, u64 r4, u64 r5)
+{
+ char *fmt = (char *) (long) r1;
+ int fmt_cnt = 0;
+ bool mod_l[3] = {};
+ int i;
+
+ /* bpf_check() guarantees that fmt points to bpf program stack and
+ * fmt_size bytes of it were initialized by bpf program
+ */
+ if (fmt[fmt_size - 1] != 0)
+ return -EINVAL;
+
+ /* check format string for allowed specifiers */
+ for (i = 0; i < fmt_size; i++)
+ if (fmt[i] == '%') {
+ if (fmt_cnt >= 3)
+ return -EINVAL;
+ i++;
+ if (i >= fmt_size)
+ return -EINVAL;
+
+ if (fmt[i] == 'l') {
+ mod_l[fmt_cnt] = true;
+ i++;
+ if (i >= fmt_size)
+ return -EINVAL;
+ } else if (fmt[i] == 'p') {
+ mod_l[fmt_cnt] = true;
+ fmt_cnt++;
+ continue;
+ }
+
+ if (fmt[i] == 'l') {
+ mod_l[fmt_cnt] = true;
+ i++;
+ if (i >= fmt_size)
+ return -EINVAL;
+ }
+
+ if (fmt[i] != 'd' && fmt[i] != 'u' && fmt[i] != 'x')
+ return -EINVAL;
+ fmt_cnt++;
+ }
+
+ return __trace_printk(1/* fake ip will not be printed */, fmt,
+ mod_l[0] ? r3 : (u32) r3,
+ mod_l[1] ? r4 : (u32) r4,
+ mod_l[2] ? r5 : (u32) r5);
+}
+
static struct bpf_func_proto kprobe_prog_funcs[] = {
[BPF_FUNC_probe_read] = {
.func = bpf_probe_read,
@@ -76,6 +130,13 @@ static struct bpf_func_proto kprobe_prog_funcs[] = {
.gpl_only = true,
.ret_type = RET_INTEGER,
},
+ [BPF_FUNC_trace_printk] = {
+ .func = bpf_trace_printk,
+ .gpl_only = true,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_PTR_TO_STACK,
+ .arg2_type = ARG_CONST_STACK_SIZE,
+ },
};

static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func_id)
@@ -90,6 +151,13 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
default:
if (func_id < 0 || func_id >= ARRAY_SIZE(kprobe_prog_funcs))
return NULL;
+
+ if (func_id == BPF_FUNC_trace_printk)
+ /* this program might be calling bpf_trace_printk,
+ * so allocate per-cpu printk buffers
+ */
+ trace_printk_init_buffers();
+
return &kprobe_prog_funcs[func_id];
}
}
--
1.7.9.5