Re: [Linaro-mm-sig] Re: [PATCH 2/2] procfs: Add 'path' to /proc/<pid>/fdinfo/
From: Kalesh Singh
Date: Tue Jun 21 2022 - 12:45:53 EST
On Wed, Jun 15, 2022 at 10:00 AM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
>
> 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.
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@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))
> > > >>> --
> > > >>> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx.
> > > >>>
> > > > _______________________________________________
> > > > Linaro-mm-sig mailing list -- linaro-mm-sig@xxxxxxxxxxxxxxxx
> > > > To unsubscribe send an email to linaro-mm-sig-leave@xxxxxxxxxxxxxxxx
> > >