Pam: avoid sleep(1) call when multithreaded

This enables PAM to be used in multithreaded mode (currently XAPI has a global lock around auth).

Using an off-cpu flamegraph I identified that concurrent PAM calls are slow due to a call to `sleep(1)`.
`pam_authenticate` calls `crypt_r` which calls `NSSLOW_Init` which on first use will try to initialize the just `dlopen`-ed library.
If it encounters a race condition it does a `sleep(1)`. This race condition can be quite reliably reproduced when performing a lot of PAM authentications from multiple threads in parallel.

GDB can also be used to confirm this by putting a breakpoint on `sleep`:
```
  #0  __sleep (seconds=seconds@entry=1) at ../sysdeps/unix/sysv/linux/sleep.c:42
  #1  0x00007ffff1548e22 in freebl_RunLoaderOnce () at lowhash_vector.c:122
  #2  0x00007ffff1548f31 in freebl_InitVector () at lowhash_vector.c:131
  #3  NSSLOW_Init () at lowhash_vector.c:148
  #4  0x00007ffff1b8f09a in __sha512_crypt_r (key=key@entry=0x7fffd8005a60 "pamtest-edvint", salt=0x7ffff31e17b8 "dIJbsXKc0",
  xapi-project#5  0x00007ffff1b8d070 in __crypt_r (key=key@entry=0x7fffd8005a60 "pamtest-edvint", salt=<optimized out>,
  xapi-project#6  0x00007ffff1dc9abc in verify_pwd_hash (p=p@entry=0x7fffd8005a60 "pamtest-edvint", hash=<optimized out>, nullok=nullok@entry=0) at passverify.c:111
  xapi-project#7  0x00007ffff1dc9139 in _unix_verify_password (pamh=pamh@entry=0x7fffd8002910, name=0x7fffd8002ab0 "pamtest-edvint", p=0x7fffd8005a60 "pamtest-edvint", ctrl=ctrl@entry=8389156) at support.c:777
  xapi-project#8  0x00007ffff1dc6556 in pam_sm_authenticate (pamh=0x7fffd8002910, flags=<optimized out>, argc=<optimized out>, argv=<optimized out>) at pam_unix_auth.c:178
  xapi-project#9  0x00007ffff7bcef1a in _pam_dispatch_aux (use_cached_chain=<optimized out>, resumed=<optimized out>, h=<optimized out>, flags=1, pamh=0x7fffd8002910) at pam_dispatch.c:110
  xapi-project#10 _pam_dispatch (pamh=pamh@entry=0x7fffd8002910, flags=1, choice=choice@entry=1) at pam_dispatch.c:426
  xapi-project#11 0x00007ffff7bce7e0 in pam_authenticate (pamh=0x7fffd8002910, flags=flags@entry=1) at pam_auth.c:34
  xapi-project#12 0x00000000005ae567 in XA_mh_authorize (username=username@entry=0x7fffd80028d0 "pamtest-edvint", password=password@entry=0x7fffd80028f0 "pamtest-edvint", error=error@entry=0x7ffff31e1be8) at xa_auth.c:83
  xapi-project#13 0x00000000005adf20 in stub_XA_mh_authorize (username=<optimized out>, password=<optimized out>) at xa_auth_stubs.c:42
```

`pam_start` and `pam_end` doesn't help here, because on `pam_end` the library is `dlclose`-ed, so on next `pam_authenticate` it will have to go through the initialization code again.
(This initialization code would've belonged into `pam_start`, not `pam_authenticate`, but there are several layers here including a call to `crypt_r`).
Upstream has fixed this problem >5 years ago by switching to libxcrypt instead.

Signed-off-by: Edwin Török <[email protected]>

ocaml/auth/dune

@@ -4,7 +4,7 @@
     (names xa_auth xa_auth_stubs)
   )
   (name pam)
-  (c_library_flags -lpam)
+  (c_library_flags -lpam -lcrypt)
   (wrapped false)
 )
 

ocaml/auth/xa_auth_stubs.c

@@ -11,8 +11,9 @@
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU Lesser General Public License for more details.
  */
-/*
- */
+
+/* must be at the beginning, it affects defines in other headers that cannot be reenabled later */
+#define _GNU_SOURCE
 
 #include <unistd.h>
 #include <stdlib.h>
@@ -70,6 +71,30 @@ CAMLprim value stub_XA_mh_chpasswd(value username, value new_password){
     CAMLreturn(ret);
 }
 
+#include <crypt.h>
+/* 'constructor' attribute will ensure this function gets call early during program startup. */
+void __attribute__((constructor)) stub_XA_workaround(void)
+{
+  struct crypt_data data;
+  memset(&data, 0, sizeof(data));
+
+  /* Initialize and load crypt library used for password hashing.
+     This library is loaded and initialized at [pam_authenticate] time and not at [pam_start].
+     If it detects a race condition (multiple threads with their own PAM contexts trying to call 'crypt_r'),
+     then it does [sleep 1] as can be seen in this call trace:
+     [pam_authenticate -> crypt_r -> __sha512_crypt_r -> freebl_InitVector -> freebl_RunLoaderOnce -> sleep].
+
+     As a workaround link with 'libcrypt' and call 'crypt_r' with a setting that will make it take tha sha512 route
+     to ensure that the library gets initialized while we're still single-threaded and stays loaded and initialized.
+
+     '$6$' is the setting for sha512 according to crypt(5).
+
+      Upstream has switched to using libxcrypt instead which doesn't have these problems, when we switch then
+      this workaround can be dropped.
+   */
+  crypt_r("", "$6$", &data);
+}
+
 /*
  * Local variables:
  * mode: C