On Sat, Jan 11, 2025 at 02:25:37AM +1100, Aleksa Sarai wrote:

On 2025-01-10, Eyal Birger [off-list ref] wrote:

quoted

Hi,

When attaching uretprobes to processes running inside docker, the attached
process is segfaulted when encountering the retprobe. The offending commit
is:

ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")

To my understanding, the reason is that now that uretprobe is a system call,
the default seccomp filters in docker block it as they only allow a specific
set of known syscalls.

FWIW, the default seccomp profile of Docker _should_ return -ENOSYS for
uretprobe (runc has a bunch of ugly logic to try to guarantee this if
Docker hasn't updated their profile to include it). Though I guess that
isn't sufficient for the magic that uretprobe(2) does...

quoted

This behavior can be reproduced by the below bash script, which works before
this commit.

Reported-by: Rafael Buchbinder <redacted>

hi,
nice ;-) thanks for the report, the problem seems to be that uretprobe syscall
is blocked and uretprobe trampoline does not expect that

I think we could add code to the uretprobe trampoline to detect this and
execute standard int3 as fallback to process uretprobe, I'm checking on that

jirka

quoted

Eyal.

--- CODE ---

#!/bin/bash

cat > /tmp/x.c << EOF
#include <stdio.h>
#include <seccomp.h>

char *syscalls[] = {
"write",
"exit_group",
};

__attribute__((noinline)) int probed(void)
{
printf("Probed\n");
return 1;
}

void apply_seccomp_filter(char **syscalls, int num_syscalls)
{
scmp_filter_ctx ctx;

ctx = seccomp_init(SCMP_ACT_ERRNO(1));
for (int i = 0; i < num_syscalls; i++) {
seccomp_rule_add(ctx, SCMP_ACT_ALLOW,
seccomp_syscall_resolve_name(syscalls[i]), 0);
}
seccomp_load(ctx);
seccomp_release(ctx);
}

int main(int argc, char *argv[])
{
int num_syscalls = sizeof(syscalls) / sizeof(syscalls[0]);

apply_seccomp_filter(syscalls, num_syscalls);

probed();

return 0;
}
EOF

cat > /tmp/trace.bt << EOF
uretprobe:/tmp/x:probed
{
    printf("ret=%d\n", retval);
}
EOF

gcc -o /tmp/x /tmp/x.c -lseccomp

/usr/bin/bpftrace /tmp/trace.bt &

sleep 5 # wait for uretprobe attach
/tmp/x

pkill bpftrace

rm /tmp/x /tmp/x.c /tmp/trace.bt

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/