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

[Christian König]

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.

Probably best to ping some core fs developer before going further with it.

BTW: Any preferred branch to push this upstream? If not I can take it 
through drm-misc-next.

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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