fix: starter: overwrite glibc's internal tid cache on clone()

Adapted from: opencontainers/runc#4247

Execution of a container using a PID namespace can fail on certain
versions of glibc when Singularity is built with Go 1.22.

This is due to Go 1.22 performing calls using pthread_self which,
from glibc 2.25, is not updated for the current TID on clone.

Fixes sylabs#2677

-----

Original runc explanation:

Since glibc 2.25, the thread-local cache of the current TID is no
longer updated in the child when calling clone(2). This results in
very unfortunate behaviour when Go does pthread calls using
pthread_self(), which has the wrong TID stored.

The "simple" solution is to forcefully overwrite this cached value.
Unfortunately (and unsurprisingly), the layout of "struct pthread"
is strictly private and can change without warning.

Luckily, glibc (currently) uses CLONE_CHILD_CLEARTID for all forks
(with the child_tid set to the cached &PTHREAD_SELF->tid), meaning
that as long as runc is using glibc, when "runc init" is spawned
the child process will have a pointer directly to the cached value
we want to change. With CONFIG_CHECKPOINT_RESTORE=y kernels on
Linux 3.5 and later, we can simply use prctl(PR_GET_TID_ADDRESS).
For older kernels we need to memory scan the TLS structure
(pthread_self() returns a pointer to the start of the structure
so we can "just" scan it for a field containing the current TID
and assume that it is the correct field).

Obviously this is all very horrific, and if you are reading this
in the future, it almost certainly has caused some horrific bug
that I did not forsee. Sorry about that. As far as I can tell,
there is no other workable solution that doesn't also depend on the
CLONE_CHILD_CLEARTID behaviour of glibc in some way. We cannot
"just" do a re-exec after clone(2) for security reasons.

Fixes opencontainers/runc#4233
Signed-off-by: Aleksa Sarai cyphar@cyphar.com

LICENSE_THIRD_PARTY.md

@@ -607,3 +607,31 @@ Contain code from the docker cli project, under the Apache License, Version 2.0.
    limitations under the License.
 */
 ```
+
+## github.com/containers/runc
+
+The source file:
+
+* `cmd/starter/c/starter.c`
+
+Contain code from the runc project, under the Apache License, Version 2.0.
+
+``` text
+runc
+
+Copyright 2012-2015 Docker, Inc.
+
+This product includes software developed at Docker, Inc. (http://www.docker.com).
+
+The following is courtesy of our legal counsel:
+
+
+Use and transfer of Docker may be subject to certain restrictions by the
+United States and other governments.
+It is your responsibility to ensure that your use and/or transfer does not
+violate applicable laws.
+
+For more information, please see http://www.bis.doc.gov
+
+See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.
+```