Processes can pin shared memory by keeping a handle to it through a file descriptor; for instance dmabufs, memfd, and ashmem (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.
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There was some concern about exposing the file path in the RFC[2], to that effect the change was split into separte patches. Also retrieving the file path from fdinfo is guarded by the same capability (PTRACE_MODE_READ) as /proc/<pid>/maps which also exposes file path, so this may not be an issue.
[1] https://lore.kernel.org/r/20210308170651.919148-1-kaleshsingh@google.com/ [2] https://lore.kernel.org/r/20220519214021.3572840-1-kaleshsingh@google.com/
Kalesh Singh (2): procfs: Add 'size' to /proc/<pid>/fdinfo/ procfs: Add 'path' to /proc/<pid>/fdinfo/
Documentation/filesystems/proc.rst | 22 ++++++++++++++++++++-- drivers/dma-buf/dma-buf.c | 1 - fs/proc/fd.c | 13 +++++++++---- 3 files changed, 29 insertions(+), 7 deletions(-)
base-commit: 8ab2afa23bd197df47819a87f0265c0ac95c5b6a