Re: [RFC PATCH] procfs: Add file path and size to /proc//fdinfo

Christian König
2022-05-20T06:29:37Z

Am 19.05.22 um 23:40 schrieb Kalesh Singh:
> Processes can pin shared memory by keeping a handle to it through a
> file descriptor; for instance dmabufs, memfd, and ashsmem (in Android).
>
> In the case of a memory leak, to identify the process pinning the
> memory, userspace needs to:
> - Iterate the /proc/<pid>/fd/* for each process
> - Do a readlink on each entry to identify the type of memory from
> the file path.
> - stat() each entry to get the size of the memory.
>
> The file permissions on /proc/<pid>/fd/* only allows for the owner
> or root to perform the operations above; and so is not suitable for
> capturing the system-wide state in a production environment.
>
> This issue was addressed for dmabufs by making /proc/*/fdinfo/*
> accessible to a process with PTRACE_MODE_READ_FSCREDS credentials[1]
> To allow the same kind of tracking for other types of shared memory,
> add the following fields to /proc/<pid>/fdinfo/<fd>:
>
> path - This allows identifying the type of memory based on common
> prefixes: e.g. "/memfd...", "/dmabuf...", "/dev/ashmem..."
>
> This was not an issued when dmabuf tracking was introduced
> because the exp_name field of dmabuf fdinfo could be used
> to distinguish dmabuf fds from other types.
>
> size - To track the amount of memory that is being pinned.
>
> dmabufs expose size as an additional field in fdinfo. Remove
> this and make it a common field for all fds.
>
> Access to /proc/<pid>/fdinfo is governed by PTRACE_MODE_READ_FSCREDS
> -- the same as for /proc/<pid>/maps which also exposes the path and
> size for mapped memory regions.
>
> This allows for a system process with PTRACE_MODE_READ_FSCREDS to
> account the pinned per-process memory via fdinfo.

I think this should be split into two patches, one adding the size and
one adding the path.

Adding the size is completely unproblematic, but the path might raise
some eyebrows.

>
> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20210308170651.919148-1-kaleshsingh%40google.com%2F&amp;data=05%7C01%7Cchristian.koenig%40amd.com%7C95ee7bf71c2c4aa342fa08da39e03398%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637885932392014544%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=kf%2B2es12hV3z5zjOFhx3EyxI1XEMeHexqTLNpNoDhAY%3D&amp;reserved=0
>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---
> Documentation/filesystems/proc.rst | 22 ++++++++++++++++++++--
> drivers/dma-buf/dma-buf.c | 1 -
> fs/proc/fd.c | 9 +++++++--
> 3 files changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 061744c436d9..ad66d78aca51 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1922,13 +1922,16 @@ 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 four fields -- 'pos', 'flags', 'mnt_id' and 'ino'.
> +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.
> +the file, 'size' represents the size of the file in bytes, and 'path'
> +represents the file path.
>
> A typical output is::
>
> @@ -1936,6 +1939,8 @@ A typical output is::
> flags: 0100002
> mnt_id: 19
> ino: 63107
> + size: 0
> + path: /dev/null
>
> All locks associated with a file descriptor are shown in its fdinfo too::
>
> @@ -1953,6 +1958,8 @@ Eventfd files
> flags: 04002
> mnt_id: 9
> ino: 63107
> + size: 0
> + path: anon_inode:[eventfd]
> eventfd-count: 5a
>
> where 'eventfd-count' is hex value of a counter.
> @@ -1966,6 +1973,8 @@ Signalfd files
> flags: 04002
> mnt_id: 9
> ino: 63107
> + size: 0
> + path: anon_inode:[signalfd]
> sigmask: 0000000000000200
>
> where 'sigmask' is hex value of the signal mask associated
> @@ -1980,6 +1989,8 @@ Epoll files
> flags: 02
> 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,
> @@ -1998,6 +2009,8 @@ For inotify files the format is the following::
> flags: 02000000
> 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
> @@ -2021,6 +2034,8 @@ For fanotify files the format is::
> flags: 02
> 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
> @@ -2046,6 +2061,8 @@ Timerfd files
> flags: 02
> mnt_id: 9
> ino: 63107
> + size: 0
> + path: anon_inode:[timerfd]
> clockid: 0
> ticks: 0
> settime flags: 01
> @@ -2070,6 +2087,7 @@ DMA Buffer files
> mnt_id: 9
> ino: 63107
> size: 32768
> + path: /dmabuf:
> count: 2
> exp_name: system-heap
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index b1e25ae98302..d61183ff3c30 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -377,7 +377,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
> {
> struct dma_buf *dmabuf = file->private_data;
>
> - seq_printf(m, "size:\t%zu\n", dmabuf->size);
> /* Don't count the temporary reference taken inside procfs seq_show */
> seq_printf(m, "count:\t%ld\n", file_count(dmabuf->file) - 1);
> seq_printf(m, "exp_name:\t%s\n", dmabuf->exp_name);
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 913bef0d2a36..a8a968bc58f0 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -54,10 +54,15 @@ static int seq_show(struct seq_file *m, void *v)
> if (ret)
> return ret;
>
> - seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\nino:\t%lu\n",
> + seq_printf(m, "pos:\t%lli\nflags:\t0%o\nmnt_id:\t%i\nino:\t%lu\nsize:\t%zu\n",
> (long long)file->f_pos, f_flags,
> real_mount(file->f_path.mnt)->mnt_id,
> - file_inode(file)->i_ino);
> + file_inode(file)->i_ino,
> + file_inode(file)->i_size);

We might consider splitting this into multiple seq_printf calls, one for
each printed attribute.

It becomes a bit unreadable and the minimal additional overhead
shouldn't matter that much.

Regards,
Christian.

> +
> + 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);
>
> base-commit: b015dcd62b86d298829990f8261d5d154b8d7af5