[RFC] procfs: Add file path and size to /proc/<pid>/fdinfo

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.

[1] https://lore.kernel.org/lkml/20210308170651.919148-1-kaleshsingh@google.com/

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(-)


base-commit: b015dcd62b86d298829990f8261d5d154b8d7af5

On Thu, May 19, 2022 at 02:40:15PM -0700, Kalesh Singh wrote:
> [...]
> +	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);

This comment implies "file" might be stale? Does that mean anything for
the above seq_file_path()?

On Thu, May 19, 2022 at 6:50 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, May 19, 2022 at 02:40:15PM -0700, Kalesh Singh wrote:
> > [...]
> > +     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);
>
> This comment implies "file" might be stale? Does that mean anything for
> the above seq_file_path()?

Hi Kees.

Thanks for taking a look. The comment above says the "files" pointer
can be stale. It doesn't affect our use of "file" here. seq_show()
takes the reference with get_file() so "file" wouldn't be destroyed
from under us.

Thanks,
Kalesh
>
> --
> Kees Cook

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

On Thu, May 19, 2022 at 11:29 PM Christian König
<christian.koenig@amd.com> wrote:
>
> 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.

Hi Christian,

Thanks for reviewing. "path" is exposed under the same ptrace
capability as in /proc/pid/maps. If we want to be more cautious, then
perhaps only adding "path" for the applicable anon inodes (dmabuf,
memfd, ...)? But prefer to keep it generic if no one sees an issue
with that.

>
> >
> > [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.

Agreed. WIll update in the next version.

Thanks,
Kalesh
>
> 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
>