Date:   Tue, 17 Dec 2019 12:05:58 +0800
From:   Wenbo Zhang <ethercflow@...il.com>
To:     Brendan Gregg <bgregg@...flix.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v12 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname

> I just realized that among my tools that want the path, the input is either:

> A) syscall tracepoints: int fd
> B) kprobes: struct file *

> This serves (A). If we ever add a different helper for (B), we might
> think that this helper was misnamed. Should it be called get_fd_path
> instead? That leaves get_file_path available for a later "struct file
> *" -> pathname helper.

+1, I'll rename it in the next version.

Brendan Gregg <bgregg@...flix.com> 于2019年12月17日周二 上午6:09写道：
>
> On Sat, Dec 14, 2019 at 8:01 PM Wenbo Zhang <ethercflow@...il.com> wrote:
> >
> > 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
> >
> > v11->v12: addressed Alexei's feedback
> > - only allow tracepoints to make sure it won't dead lock
> >
> > v10->v11: addressed Al and Alexei's feedback
> > - fix missing fput()
> >
> > v9->v10: addressed Andrii's feedback
> > - send this patch together with the patch selftests as one patch series
> >
> > v8->v9:
> > - format helper description
> >
> > v7->v8: addressed Alexei's feedback
> > - use fget_raw instead of fdget_raw, as fdget_raw is only used inside fs/
> > - ensure we're in user context which is safe fot the help to run
> > - filter unmountable pseudo filesystem, because they don't have real path
> > - supplement the description of this helper function
> >
> > v6->v7:
> > - fix missing signed-off-by line
> >
> > v5->v6: addressed Andrii's feedback
> > - avoid unnecessary goto end by having two explicit returns
> >
> > v4->v5: addressed Andrii and Daniel's feedback
> > - rename bpf_fd2path to bpf_get_file_path to be consistent with other
> > helper's names
> > - when fdget_raw fails, set ret to -EBADF instead of -EINVAL
> > - remove fdput from fdget_raw's error path
> > - use IS_ERR instead of IS_ERR_OR_NULL as d_path ether returns a pointer
> > into the buffer or an error code if the path was too long
> > - modify the normal path's return value to return copied string length
> > including NUL
> > - update this helper description's Return bits.
> >
> > v3->v4: addressed Daniel's feedback
> > - fix missing fdput()
> > - move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
> > - move fd2path's test code to another patch
> > - add comment to explain why use fdget_raw instead of fdget
> >
> > v2->v3: addressed Yonghong's feedback
> > - remove unnecessary LOCKDOWN_BPF_READ
> > - refactor error handling section for enhanced readability
> > - provide a test case in tools/testing/selftests/bpf
> >
> > v1->v2: addressed Daniel's feedback
> > - fix backward compatibility
> > - add this helper description
> > - fix signed-off name
> >
> > Signed-off-by: Wenbo Zhang <ethercflow@...il.com>
> > ---
> >  include/uapi/linux/bpf.h       | 29 +++++++++++++-
> >  kernel/trace/bpf_trace.c       | 70 ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 29 +++++++++++++-
> >  3 files changed, 126 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index dbbcf0b02970..71d9705df120 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2821,6 +2821,32 @@ union bpf_attr {
> >   *     Return
> >   *             On success, the strictly positive length of the string, including
> >   *             the trailing NUL character. On error, a negative value.
> > + *
> > + * int bpf_get_file_path(char *path, u32 size, int fd)
> > + *     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*. Notice the **path** don't support unmountable
> > + *             pseudo filesystems as they don't have path (eg: SOCKFS, PIPEFS).
> > + *             The *size* must be strictly positive. On success, the helper
> > + *             makes sure that the *path* is NUL-terminated, and the buffer
> > + *             could be:
> > + *             - a regular full path (include mountable fs eg: /proc, /sys)
> > + *             - a regular full path with "(deleted)" at the end.
> > + *             On failure, it is filled with zeroes.
> > + *     Return
> > + *             On success, returns the length of the copied string INCLUDING
> > + *             the trailing NUL.
> > + *
> > + *             On failure, the returned value is one of the following:
> > + *
> > + *             **-EPERM** if no permission to get the path (eg: in irq ctx).
> > + *
> > + *             **-EBADF** if *fd* is invalid.
> > + *
> > + *             **-EINVAL** if *fd* corresponds to a unmountable pseudo fs
> > + *
> > + *             **-ENAMETOOLONG** if full path is longer than *size*
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)          \
> >         FN(unspec),                     \
> > @@ -2938,7 +2964,8 @@ union bpf_attr {
> >         FN(probe_read_user),            \
> >         FN(probe_read_kernel),          \
> >         FN(probe_read_user_str),        \
> > -       FN(probe_read_kernel_str),
> > +       FN(probe_read_kernel_str),      \
> > +       FN(get_file_path),
> >
> >  /* 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 e5ef4ae9edb5..db9c0ec46a5d 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -762,6 +762,72 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
> >         .arg1_type      = ARG_ANYTHING,
> >  };
> >
> > +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> > +{
> > +       struct file *f;
> > +       char *p;
> > +       int ret = -EBADF;
> > +
> > +       /* Ensure we're in user context which is safe for the helper to
> > +        * run. This helper has no business in a kthread.
> > +        */
> > +       if (unlikely(in_interrupt() ||
> > +                    current->flags & (PF_KTHREAD | PF_EXITING))) {
> > +               ret = -EPERM;
> > +               goto error;
> > +       }
> > +
> > +       /* Use fget_raw instead of fget to support O_PATH, and it doesn't
> > +        * have any sleepable code, so it's ok to be here.
> > +        */
> > +       f = fget_raw(fd);
> > +       if (!f)
> > +               goto error;
> > +
> > +       /* For unmountable pseudo filesystem, it seems to have no meaning
> > +        * to get their fake paths as they don't have path, and to be no
> > +        * way to validate this function pointer can be always safe to call
> > +        * in the current context.
> > +        */
> > +       if (f->f_path.dentry->d_op && f->f_path.dentry->d_op->d_dname) {
> > +               ret = -EINVAL;
> > +               fput(f);
> > +               goto error;
> > +       }
> > +
> > +       /* After filter unmountable pseudo filesytem, d_path won't call
> > +        * dentry->d_op->d_name(), the normally path doesn't have any
> > +        * sleepable code, and despite it uses the current macro to get
> > +        * fs_struct (current->fs), we've already ensured we're in user
> > +        * context, so it's ok to be here.
> > +        */
> > +       p = d_path(&f->f_path, dst, size);
> > +       if (IS_ERR(p)) {
> > +               ret = PTR_ERR(p);
> > +               fput(f);
> > +               goto error;
> > +       }
> > +
> > +       ret = strlen(p);
> > +       memmove(dst, p, ret);
> > +       dst[ret++] = '\0';
> > +       fput(f);
> > +       return ret;
> > +
> > +error:
> > +       memset(dst, '0', size);
> > +       return ret;
> > +}
> > +
> > +static const struct bpf_func_proto bpf_get_file_path_proto = {
> > +       .func       = bpf_get_file_path,
> > +       .gpl_only   = true,
> > +       .ret_type   = RET_INTEGER,
> > +       .arg1_type  = ARG_PTR_TO_UNINIT_MEM,
> > +       .arg2_type  = ARG_CONST_SIZE,
> > +       .arg3_type  = ARG_ANYTHING,
> > +};
> > +
> >  static const struct bpf_func_proto *
> >  tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  {
> > @@ -953,6 +1019,8 @@ tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >                 return &bpf_get_stackid_proto_tp;
> >         case BPF_FUNC_get_stack:
> >                 return &bpf_get_stack_proto_tp;
> > +       case BPF_FUNC_get_file_path:
> > +               return &bpf_get_file_path_proto;
> >         default:
> >                 return tracing_func_proto(func_id, prog);
> >         }
> > @@ -1146,6 +1214,8 @@ raw_tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >                 return &bpf_get_stackid_proto_raw_tp;
> >         case BPF_FUNC_get_stack:
> >                 return &bpf_get_stack_proto_raw_tp;
> > +       case BPF_FUNC_get_file_path:
> > +               return &bpf_get_file_path_proto;
> >         default:
> >                 return tracing_func_proto(func_id, prog);
> >         }
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index dbbcf0b02970..71d9705df120 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -2821,6 +2821,32 @@ union bpf_attr {
> >   *     Return
> >   *             On success, the strictly positive length of the string, including
> >   *             the trailing NUL character. On error, a negative value.
> > + *
> > + * int bpf_get_file_path(char *path, u32 size, int fd)
> > + *     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*. Notice the **path** don't support unmountable
> > + *             pseudo filesystems as they don't have path (eg: SOCKFS, PIPEFS).
> > + *             The *size* must be strictly positive. On success, the helper
> > + *             makes sure that the *path* is NUL-terminated, and the buffer
> > + *             could be:
> > + *             - a regular full path (include mountable fs eg: /proc, /sys)
> > + *             - a regular full path with "(deleted)" at the end.
> > + *             On failure, it is filled with zeroes.
> > + *     Return
> > + *             On success, returns the length of the copied string INCLUDING
> > + *             the trailing NUL.
> > + *
> > + *             On failure, the returned value is one of the following:
> > + *
> > + *             **-EPERM** if no permission to get the path (eg: in irq ctx).
> > + *
> > + *             **-EBADF** if *fd* is invalid.
> > + *
> > + *             **-EINVAL** if *fd* corresponds to a unmountable pseudo fs
> > + *
> > + *             **-ENAMETOOLONG** if full path is longer than *size*
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)          \
> >         FN(unspec),                     \
> > @@ -2938,7 +2964,8 @@ union bpf_attr {
> >         FN(probe_read_user),            \
> >         FN(probe_read_kernel),          \
> >         FN(probe_read_user_str),        \
> > -       FN(probe_read_kernel_str),
> > +       FN(probe_read_kernel_str),      \
> > +       FN(get_file_path),
>
>
> I just realized that among my tools that want the path, the input is either:
>
> A) syscall tracepoints: int fd
> B) kprobes: struct file *
>
> This serves (A). If we ever add a different helper for (B), we might
> think that this helper was misnamed. Should it be called get_fd_path
> instead? That leaves get_file_path available for a later "struct file
> *" -> pathname helper.
>
> Brendan
>
> --
> Brendan Gregg, Senior Performance Architect, Netflix

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