We only use the last pid from the list in NSpid entry (from
/proc/<pid>/fdinfo/<pidfd>) while restoring pidfds.
The last pid refers to the pid of the process in the most deeply nested
pid namespace. Since CRIU does not currently support nested pid
namespaces, this entry is the one we want.

After Linux 6.9, inode numbers can be used to compare pidfds. pidfds
referring to the same process will have the same inode numbers. We use
inode numbers to restore pidfds that point to dead processes.

Signed-off-by: Bhavik Sachdev <[email protected]>

Process file descriptors (pidfds) were introduced to provide a stable
handle on a process. They solve the problem of pid recycling.

For a detailed explanation, see https://lwn.net/Articles/801319/ and
http://www.corsix.org/content/what-is-a-pidfd

Before Linux 6.9, anonymous inodes were used for the implementation of
pidfds. So, we detect them in a fashion similiar to other fd types that
use anonymous inodes by calling `readlink()`.
After 6.9, pidfs (a file system for pidfds) was introduced.
In 6.9 `S_ISREG()` returned true for pidfds, but this again changed with
6.10.
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/pidfs.c?h=v6.11-rc2#n285)
After this change, pidfs inodes have no file type in st_mode in
userspace.
We use `PID_FS_MAGIC` to detect pidfds for kernel >= 6.9
Hence, check for pidfds occurs before the check for regular files.

For pidfds that refer to dead processes, we lose the pid of the process
as the Pid and NSpid fields in /proc/<pid>/fdinfo/<pidfd> change to -1.
So, we create a temporary process for each unique inode and open pidfds
that refer to this process. After all pidfds have been opened we kill
this temporary process.

This commit does not include support for pidfds that point to a specific
thread, i.e pidfds opened with `PIDFD_THREAD` flag.

Fixes: checkpoint-restore#2258

Signed-off-by: Bhavik Sachdev <[email protected]>

Ensures that entries in /proc/<pid>/fdinfo/<pidfd> are same.

Signed-off-by: Bhavik Sachdev <[email protected]>

Ensure `pidfd_send_signal()` syscall works as expected after C/R.

Signed-off-by: Bhavik Sachdev <[email protected]>

Validate that pidfds can been used to send signals to different
processes after C/R using the `pidfd_send_signal()` syscall.

Signed-off-by: Bhavik Sachdev <[email protected]>

After, C/R of pidfds that point to dead processes their inodes might
change. But if two pidfds point to same dead process they should
continue to do so after C/R.

This test ensures that this happens by calling `statx()` on pidfds after
C/R and then comparing their inode numbers.

Support for comparing pidfds by using `statx()` and inode numbers was
introduced alongside pidfs. So if `f_type` of pidfd is not equal to
`PID_FS_MAGIC` then we skip this test.

signed-off-by: Bhavik Sachdev <[email protected]>

We get the read end of a pipe using `pidfd_getfd` and check if we can
read from it after C/R.

signed-off-by: Bhavik Sachdev <[email protected]>

We open a pidfd to a thread using `PIDFD_THREAD` flag and after C/R
ensure that we can send signals using it with `PIDFD_SIGNAL_THREAD`.

signed-off-by: Bhavik Sachdev <[email protected]>