Re: [PATCH bpf-next v4] bpf: add new helper fd2path for mapping a file descriptor to a pathname

Also I'd prefer fdget_*()'s error path not calling fdput(f).

Thank you, I've committed a new patch to fix this.

Daniel Borkmann [off-list ref] 于2019年10月30日周三 下午11:32写道：

On Tue, Oct 29, 2019 at 11:48:44AM -0700, Andrii Nakryiko wrote:

quoted

On Mon, Oct 28, 2019 at 1:59 PM Wenbo Zhang [off-list ref] wrote:

quoted

When people want to identify which file system files are being opened,
read, and written to, they can use this helper with file descriptor as
input to achieve this goal. Other pseudo filesystems are also supported.

This requirement is mainly discussed here:

  https://github.com/iovisor/bcc/issues/237

v3->v4:
- fix missing fdput()
- move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
- move fd2path's test code to another patch

v2->v3:
- remove unnecessary LOCKDOWN_BPF_READ
- refactor error handling section for enhanced readability
- provide a test case in tools/testing/selftests/bpf

v1->v2:
- fix backward compatibility
- add this helper description
- fix signed-off name

Signed-off-by: Wenbo Zhang <redacted>
---
 include/uapi/linux/bpf.h       | 14 +++++++++++-
 kernel/trace/bpf_trace.c       | 40 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 14 +++++++++++-
 3 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 4af8b0819a32..124632b2a697 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -2775,6 +2775,17 @@ union bpf_attr {
  *             restricted to raw_tracepoint bpf programs.
  *     Return
  *             0 on success, or a negative error in case of failure.
+ *
+ * int bpf_fd2path(char *path, u32 size, int fd)

from what I can see, we don't have any BPF helper with this naming
approach(2 -> to, 4 -> for, etc). How about something like
bpf_get_file_path?

quoted

+ *     Description
+ *             Get **file** atrribute from the current task by *fd*, then call
+ *             **d_path** to get it's absolute path and copy it as string into
+ *             *path* of *size*. The **path** also support pseudo filesystems
+ *             (whether or not it can be mounted). The *size* must be strictly
+ *             positive. On success, the helper makes sure that the *path* is
+ *             NUL-terminated. On failure, it is filled with zeroes.
+ *     Return
+ *             0 on success, or a negative error in case of failure.

Mention that we actually return a positive number on success, which is
a size of the string + 1 for NUL byte (the +1 is not true right now,
but I think should be).

quoted

  */
 #define __BPF_FUNC_MAPPER(FN)          \
        FN(unspec),                     \

@@ -2888,7 +2899,8 @@ union bpf_attr {
        FN(sk_storage_delete),          \
        FN(send_signal),                \
        FN(tcp_gen_syncookie),          \
-       FN(skb_output),
+       FN(skb_output),                 \
+       FN(fd2path),

 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 571c25d60710..dd7b070df3d6 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -683,6 +683,44 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
        .arg1_type      = ARG_ANYTHING,
 };

+BPF_CALL_3(bpf_fd2path, char *, dst, u32, size, int, fd)
+{
+       struct fd f;
+       char *p;
+       int ret = -EINVAL;
+
+       /* Use fdget_raw instead of fdget to support O_PATH */
+       f = fdget_raw(fd);

I haven't followed previous discussions, so sorry if this was asked
before. Can either fdget_raw or d_path sleep? Also, d_path seems to be
relying on current, which in the interrupt context might not be what
you really want. Have you considered these problems?

quoted

+       if (!f.file)
+               goto error;
+
+       p = d_path(&f.file->f_path, dst, size);
+       if (IS_ERR_OR_NULL(p)) {
+               ret = PTR_ERR(p);

if p can really be NULL, you'd get ret == 0 here, which is probably
not what you want.
But reading d_path, it seems like it's either valid pointer or error,
so just use IS_ERR above?

quoted

+               goto error;
+       }
+
+       ret = strlen(p);
+       memmove(dst, p, ret);
+       dst[ret] = '\0';

I think returning number of useful bytes (including terminating NUL)
is good and follows bpf_probe_read_str() convention. So ret++ here?

quoted

+       goto end;
+
+error:
+       memset(dst, '0', size);
+end:
+       fdput(f);

Also I'd prefer fdget_*()'s error path not calling fdput(f).

quoted

quoted

+       return ret;
+}
+

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