Message-ID: <CAC_TJvfWos07gCJ2V8cdp29QKSgJrXv5g9b_jGfg42c6f8simw@mail.gmail.com>
Date:   Tue, 21 Jun 2022 09:45:23 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Christian König <ckoenig.leichtzumerken@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        Ioannis Ilkos <ilkos@...gle.com>,
        "T.J. Mercier" <tjmercier@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Jonathan Corbet <corbet@....net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Christoph Anton Mitterer <mail@...istoph.anton.mitterer.name>,
        Johannes Weiner <hannes@...xchg.org>,
        Colin Cross <ccross@...gle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [Linaro-mm-sig] Re: [PATCH 2/2] procfs: Add 'path' to /proc/<pid>/fdinfo/

On Wed, Jun 15, 2022 at 10:00 AM Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
> On Wed, Jun 1, 2022 at 8:31 PM Kalesh Singh <kaleshsingh@...gle.com> wrote:
> >
> > On Wed, Jun 1, 2022 at 8:02 AM Christian König
> > <ckoenig.leichtzumerken@...il.com> wrote:
> > >
> > > Am 01.06.22 um 00:48 schrieb Stephen Brennan:
> > > > Kalesh Singh <kaleshsingh@...gle.com> writes:
> > > >> On Tue, May 31, 2022 at 3:07 PM Stephen Brennan
> > > >> <stephen.s.brennan@...cle.com> 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.

Hi all,

If there are no objections to this, then I plan to respin the patch
for just anonymous inodes. Please let me know if there are further
concerns.

Thanks,
Kalesh

>
> 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@...gle.com>
> > > >>>> ---
> > > >>>>
> > > >>>> 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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> > > >>>
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