Re: [Linaro-mm-sig] Re: [PATCH 2/2] procfs: Add 'path' to /proc/<pid>/fdinfo/

[Kalesh Singh]

On Wed, Jun 1, 2022 at 8:31 PM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
>
> On Wed, Jun 1, 2022 at 8:02 AM Christian König
> <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> >
> > Am 01.06.22 um 00:48 schrieb Stephen Brennan:
> > > Kalesh Singh <kaleshsingh@xxxxxxxxxx> writes:
> > >> On Tue, May 31, 2022 at 3:07 PM Stephen Brennan
> > >> <stephen.s.brennan@xxxxxxxxxx> wrote:
> > >>> On 5/31/22 14:25, Kalesh Singh wrote:
> > >>>> In order to identify the type of memory a process has pinned through
> > >>>> its open fds, add the file path to fdinfo output. This allows
> > >>>> identifying memory types based on common prefixes. e.g. "/memfd...",
> > >>>> "/dmabuf...", "/dev/ashmem...".
> > >>>>
> > >>>> Access to /proc/<pid>/fdinfo is governed by PTRACE_MODE_READ_FSCREDS
> > >>>> the same as /proc/<pid>/maps which also exposes the file path of
> > >>>> mappings; so the security permissions for accessing path is consistent
> > >>>> with that of /proc/<pid>/maps.
> > >>> Hi Kalesh,
> > >> Hi Stephen,
> > >>
> > >> Thanks for taking a look.
> > >>
> > >>> I think I see the value in the size field, but I'm curious about path,
> > >>> which is available via readlink /proc/<pid>/fd/<n>, since those are
> > >>> symlinks to the file themselves.
> > >> This could work if we are root, but the file permissions wouldn't
> > >> allow us to do the readlink on other processes otherwise. We want to
> > >> be able to capture the system state in production environments from
> > >> some trusted process with ptrace read capability.
> > > Interesting, thanks for explaining. It seems weird to have a duplicate
> > > interface for the same information but such is life.
> >
> > Yeah, the size change is really straight forward but for this one I'm
> > not 100% sure either.
>
> The 2 concerns I think are:
>   1. Fun characters in the path names
>   2. If exposing the path is appropriate to begin with.
>
> One way I think we can address both is to only expose the path for
> anon inodes. Then we have well-known path formats and we don't expose
> much about which files a process is accessing since these aren't real
> paths.
>
> +       if (is_anon_inode(inode)) {
> +               seq_puts(m, "path:\t");
> +               seq_file_path(m, file, "\n");
> +               seq_putc(m, '\n');
> +       }
>
> Interested to hear thoughts on it.

Adding Christoph,

To be able to identify types of shared memory processes pin through
FDs in production builds, we would like to add a 'path' field to
fdinfo of anon inodes. We could then use the common prefixes
("/dmabuf", "/memfd", ...) to identify different types.

Would appreciate any feedback from the FS perspective.

Thanks,
Kalesh

>
> >
> > Probably best to ping some core fs developer before going further with it.
>
> linux-fsdevel is cc'd here. Adding Al Vrio as well. Please let me know
> if there are other parties I should include.
>
> >
> > BTW: Any preferred branch to push this upstream? If not I can take it
> > through drm-misc-next.
>
> No other dependencies for this, so drm-misc-next is good.
>
> Thanks,
> Kalesh
>
> >
> > Regards,
> > Christian.
> >
> > >
> > >>> File paths can contain fun characters like newlines or colons, which
> > >>> could make parsing out filenames in this text file... fun. How would your
> > >>> userspace parsing logic handle "/home/stephen/filename\nsize:\t4096"? The
> > >>> readlink(2) API makes that easy already.
> > >> I think since we have escaped the "\n" (seq_file_path(m, file, "\n")),
> > > I really should have read through that function before commenting,
> > > thanks for teaching me something new :)
> > >
> > > Stephen
> > >
> > >> then user space might parse this line like:
> > >>
> > >> if (strncmp(line, "path:\t", 6) == 0)
> > >>          char* path = line + 6;
> > >>
> > >>
> > >> Thanks,
> > >> Kalesh
> > >>
> > >>> Is the goal avoiding races (e.g. file descriptor 3 is closed and reopened
> > >>> to a different path between reading fdinfo and stating the fd)?
> > >>>
> > >>> Stephen
> > >>>
> > >>>> Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > >>>> ---
> > >>>>
> > >>>> Changes from rfc:
> > >>>>    - Split adding 'size' and 'path' into a separate patches, per Christian
> > >>>>    - Fix indentation (use tabs) in documentaion, per Randy
> > >>>>
> > >>>>   Documentation/filesystems/proc.rst | 14 ++++++++++++--
> > >>>>   fs/proc/fd.c                       |  4 ++++
> > >>>>   2 files changed, 16 insertions(+), 2 deletions(-)
> > >>>>
> > >>>> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > >>>> index 779c05528e87..591f12d30d97 100644
> > >>>> --- a/Documentation/filesystems/proc.rst
> > >>>> +++ b/Documentation/filesystems/proc.rst
> > >>>> @@ -1886,14 +1886,16 @@ if precise results are needed.
> > >>>>   3.8  /proc/<pid>/fdinfo/<fd> - Information about opened file
> > >>>>   ---------------------------------------------------------------
> > >>>>   This file provides information associated with an opened file. The regular
> > >>>> -files have at least five fields -- 'pos', 'flags', 'mnt_id', 'ino', and 'size'.
> > >>>> +files have at least six fields -- 'pos', 'flags', 'mnt_id', 'ino', 'size',
> > >>>> +and 'path'.
> > >>>>
> > >>>>   The 'pos' represents the current offset of the opened file in decimal
> > >>>>   form [see lseek(2) for details], 'flags' denotes the octal O_xxx mask the
> > >>>>   file has been created with [see open(2) for details] and 'mnt_id' represents
> > >>>>   mount ID of the file system containing the opened file [see 3.5
> > >>>>   /proc/<pid>/mountinfo for details]. 'ino' represents the inode number of
> > >>>> -the file, and 'size' represents the size of the file in bytes.
> > >>>> +the file, 'size' represents the size of the file in bytes, and 'path'
> > >>>> +represents the file path.
> > >>>>
> > >>>>   A typical output is::
> > >>>>
> > >>>> @@ -1902,6 +1904,7 @@ A typical output is::
> > >>>>        mnt_id: 19
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   /dev/null
> > >>>>
> > >>>>   All locks associated with a file descriptor are shown in its fdinfo too::
> > >>>>
> > >>>> @@ -1920,6 +1923,7 @@ Eventfd files
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:[eventfd]
> > >>>>        eventfd-count:  5a
> > >>>>
> > >>>>   where 'eventfd-count' is hex value of a counter.
> > >>>> @@ -1934,6 +1938,7 @@ Signalfd files
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:[signalfd]
> > >>>>        sigmask:        0000000000000200
> > >>>>
> > >>>>   where 'sigmask' is hex value of the signal mask associated
> > >>>> @@ -1949,6 +1954,7 @@ Epoll files
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:[eventpoll]
> > >>>>        tfd:        5 events:       1d data: ffffffffffffffff pos:0 ino:61af sdev:7
> > >>>>
> > >>>>   where 'tfd' is a target file descriptor number in decimal form,
> > >>>> @@ -1968,6 +1974,7 @@ For inotify files the format is the following::
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:inotify
> > >>>>        inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d
> > >>>>
> > >>>>   where 'wd' is a watch descriptor in decimal form, i.e. a target file
> > >>>> @@ -1992,6 +1999,7 @@ For fanotify files the format is::
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:[fanotify]
> > >>>>        fanotify flags:10 event-flags:0
> > >>>>        fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
> > >>>>        fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4
> > >>>> @@ -2018,6 +2026,7 @@ Timerfd files
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   0
> > >>>> +     path:   anon_inode:[timerfd]
> > >>>>        clockid: 0
> > >>>>        ticks: 0
> > >>>>        settime flags: 01
> > >>>> @@ -2042,6 +2051,7 @@ DMA Buffer files
> > >>>>        mnt_id: 9
> > >>>>        ino:    63107
> > >>>>        size:   32768
> > >>>> +     path:   /dmabuf:
> > >>>>        count:  2
> > >>>>        exp_name:  system-heap
> > >>>>
> > >>>> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> > >>>> index 464bc3f55759..8889a8ba09d4 100644
> > >>>> --- a/fs/proc/fd.c
> > >>>> +++ b/fs/proc/fd.c
> > >>>> @@ -60,6 +60,10 @@ static int seq_show(struct seq_file *m, void *v)
> > >>>>        seq_printf(m, "ino:\t%lu\n", file_inode(file)->i_ino);
> > >>>>        seq_printf(m, "size:\t%lli\n", (long long)file_inode(file)->i_size);
> > >>>>
> > >>>> +     seq_puts(m, "path:\t");
> > >>>> +     seq_file_path(m, file, "\n");
> > >>>> +     seq_putc(m, '\n');
> > >>>> +
> > >>>>        /* show_fd_locks() never deferences files so a stale value is safe */
> > >>>>        show_fd_locks(m, file, files);
> > >>>>        if (seq_has_overflowed(m))
> > >>> --
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