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4988e35e95Now every time the daemon reads an open request, it gets a new anonymous fd
and ondemand_id. With the introduction of "restore", it is possible to read
the same open request more than once, and therefore an object can have more
than one anonymous fd.
If the anonymous fd is not unique, the following concurrencies will result
in an fd leak:
t1 | t2 | t3
------------------------------------------------------------
cachefiles_ondemand_init_object
cachefiles_ondemand_send_req
REQ_A = kzalloc(sizeof(*req) + data_len)
wait_for_completion(&REQ_A->done)
cachefiles_daemon_read
cachefiles_ondemand_daemon_read
REQ_A = cachefiles_ondemand_select_req
cachefiles_ondemand_get_fd
load->fd = fd0
ondemand_id = object_id0
------ restore ------
cachefiles_ondemand_restore
// restore REQ_A
cachefiles_daemon_read
cachefiles_ondemand_daemon_read
REQ_A = cachefiles_ondemand_select_req
cachefiles_ondemand_get_fd
load->fd = fd1
ondemand_id = object_id1
process_open_req(REQ_A)
write(devfd, ("copen %u,%llu", msg->msg_id, size))
cachefiles_ondemand_copen
xa_erase(&cache->reqs, id)
complete(&REQ_A->done)
kfree(REQ_A)
process_open_req(REQ_A)
// copen fails due to no req
// daemon close(fd1)
cachefiles_ondemand_fd_release
// set object closed
-- umount --
cachefiles_withdraw_cookie
cachefiles_ondemand_clean_object
cachefiles_ondemand_init_close_req
if (!cachefiles_ondemand_object_is_open(object))
return -ENOENT;
// The fd0 is not closed until the daemon exits.
However, the anonymous fd holds the reference count of the object and the
object holds the reference count of the cookie. So even though the cookie
has been relinquished, it will not be unhashed and freed until the daemon
exits.
In fscache_hash_cookie(), when the same cookie is found in the hash list,
if the cookie is set with the FSCACHE_COOKIE_RELINQUISHED bit, then the new
cookie waits for the old cookie to be unhashed, while the old cookie is
waiting for the leaked fd to be closed, if the daemon does not exit in time
it will trigger a hung task.
To avoid this, allocate a new anonymous fd only if no anonymous fd has
been allocated (ondemand_id == 0) or if the previously allocated anonymous
fd has been closed (ondemand_id == -1). Moreover, returns an error if
ondemand_id is valid, letting the daemon know that the current userland
restore logic is abnormal and needs to be checked.
Fixes:
|
History
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| ipc | ||
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.