%s specifier makes bpf program and kernel debugging easier.
To make sure that trace_printk won't crash the unsafe string
is copied into stack and unsafe pointer is substituted.
String is also sanitized for printable characters.
The cost of swapping FS in probe_kernel_read is
amortized over 4 chars to improve performance.
Suggested-by: Brendan Gregg <brendan.d.gr...@gmail.com>
Signed-off-by: Alexei Starovoitov <a...@plumgrid.com>
---
The following C program:
#include <linux/fs.h>
int foo(struct pt_regs *ctx, struct filename *filename)
{
void *name = 0;
bpf_probe_read(&name, sizeof(name), &filename->name);
bpf_trace_printk("executed %s\\n", name);
return 0;
}
when attached to kprobe do_execve()
will produce output in /sys/kernel/debug/tracing/trace_pipe :
make-13492 [002] d..1 3250.997277: : executed /bin/sh
sh-13493 [004] d..1 3250.998716: : executed /usr/bin/gcc
gcc-13494 [002] d..1 3250.999822: : executed
/usr/lib/gcc/x86_64-linux-gnu/4.7/cc1
gcc-13495 [002] d..1 3251.006731: : executed /usr/bin/as
gcc-13496 [002] d..1 3251.011831: : executed
/usr/lib/gcc/x86_64-linux-gnu/4.7/collect2
collect2-13497 [000] d..1 3251.012941: : executed /usr/bin/ld
kernel/trace/bpf_trace.c | 60 +++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 57 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index ef9936df1b04..60d8f95258ed 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -79,13 +79,51 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
.arg3_type = ARG_ANYTHING,
};
+static bool is_valid_char(char c)
+{
+ return (isprint(c) || isspace(c)) && isascii(c);
+}
+
+/* similar to strncpy_from_user() but with extra checks */
+static void probe_read_string(char *buf, int size, long unsafe_ptr)
+{
+ char dst[4];
+ int i = 0;
+
+ size--;
+ for (;;) {
+ if (probe_kernel_read(dst, (void *) unsafe_ptr, 4))
+ break;
+
+ unsafe_ptr += 4;
+
+ if (dst[0] == 0 || !is_valid_char(dst[0]) || i >= size)
+ break;
+ buf[i++] = dst[0];
+
+ if (dst[1] == 0 || !is_valid_char(dst[1]) || i >= size)
+ break;
+ buf[i++] = dst[1];
+
+ if (dst[2] == 0 || !is_valid_char(dst[2]) || i >= size)
+ break;
+ buf[i++] = dst[2];
+
+ if (dst[3] == 0 || !is_valid_char(dst[3]) || i >= size)
+ break;
+ buf[i++] = dst[3];
+ }
+ buf[i] = 0;
+}
+
/*
* limited trace_printk()
- * only %d %u %x %ld %lu %lx %lld %llu %llx %p conversion specifiers allowed
+ * only %d %u %x %ld %lu %lx %lld %llu %llx %p %s conversion specifiers allowed
*/
static u64 bpf_trace_printk(u64 r1, u64 fmt_size, u64 r3, u64 r4, u64 r5)
{
char *fmt = (char *) (long) r1;
+ char buf[64];
int mod[3] = {};
int fmt_cnt = 0;
int i;
@@ -100,7 +138,7 @@ static u64 bpf_trace_printk(u64 r1, u64 fmt_size, u64 r3,
u64 r4, u64 r5)
/* check format string for allowed specifiers */
for (i = 0; i < fmt_size; i++) {
- if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i]))
+ if (!is_valid_char(fmt[i]))
return -EINVAL;
if (fmt[i] != '%')
@@ -114,12 +152,28 @@ static u64 bpf_trace_printk(u64 r1, u64 fmt_size, u64 r3,
u64 r4, u64 r5)
if (fmt[i] == 'l') {
mod[fmt_cnt]++;
i++;
- } else if (fmt[i] == 'p') {
+ } else if (fmt[i] == 'p' || fmt[i] == 's') {
mod[fmt_cnt]++;
i++;
if (!isspace(fmt[i]) && !ispunct(fmt[i]) && fmt[i] != 0)
return -EINVAL;
fmt_cnt++;
+ if (fmt[i - 1] == 's') {
+ switch (fmt_cnt) {
+ case 1:
+ probe_read_string(buf, sizeof(buf), r3);
+ r3 = (long) buf;
+ break;
+ case 2:
+ probe_read_string(buf, sizeof(buf), r4);
+ r4 = (long) buf;
+ break;
+ case 3:
+ probe_read_string(buf, sizeof(buf), r5);
+ r5 = (long) buf;
+ break;
+ }
+ }
continue;
}
--
1.7.9.5
--
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