Re: [PATCH bpf v2 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

["Daniel Xu" <dxu@dxuuu.xyz>]

On Thu Nov 5, 2020 at 3:31 PM PST, Song Liu wrote:
>
>
> > On Nov 5, 2020, at 3:22 PM, Daniel Xu <dxu@dxuuu.xyz> wrote:
> > 
> > On Thu Nov 5, 2020 at 1:32 PM PST, Andrii Nakryiko wrote:
> >> On Wed, Nov 4, 2020 at 8:51 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> > [...]
> >>> diff --git a/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> >>> new file mode 100644
> >>> index 000000000000..41c3e296566e
> >>> --- /dev/null
> >>> +++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> >>> @@ -0,0 +1,34 @@
> >>> +// SPDX-License-Identifier: GPL-2.0
> >>> +
> >>> +#include <linux/bpf.h>
> >>> +#include <bpf/bpf_helpers.h>
> >>> +#include <bpf/bpf_tracing.h>
> >>> +
> >>> +#include <sys/types.h>
> >>> +
> >>> +struct sys_enter_write_args {
> >>> +       unsigned long long pad;
> >>> +       int syscall_nr;
> >>> +       int pad1; /* 4 byte hole */
> >> 
> >> I have a hunch that this explicit padding might break on big-endian
> >> architectures?..
> >> 
> >> Can you instead include "vmlinux.h" in this file and use struct
> >> trace_event_raw_sys_enter? you'll just need ctx->args[2] to get that
> >> buffer pointer.
> >> 
> >> Alternatively, and it's probably simpler overall would be to just
> >> provide user-space pointer through global variable:
> >> 
> >> void *user_ptr;
> >> 
> >> 
> >> bpf_probe_read_user_str(buf, ..., user_ptr);
> >> 
> >> From user-space:
> >> 
> >> skel->bss->user_ptr = &my_userspace_buf;
> >> 
> >> Full control. You can trigger tracepoint with just an usleep(1), for
> >> instance.
> > 
> > Yeah, that sounds better. I'll send a v4 with passing a ptr.
> > 
> > Thanks,
> > Daniel
>
> One more comment, how about we test multiple strings with different
> lengths? In this way, we can catch other alignment issues.

Sure, will do that in v4 also.