testsuite: explain why overriding function may fail
On my computer `testsuite/test-modprobe modprobe_install_cmd_loop` was
failing because when it forks off the shell the child process ends up
calling syscall() which are are supposed to wrap. Here's the backtrace:
#0 0x00007ffff6fdb66b in raise () from /lib64/libc.so.6
#1 0x00007ffff6fdd381 in abort () from /lib64/libc.so.6
#2 0x00007ffff77bac97 in syscall (__sysno=<optimized out>)
at testsuite/init_module.c:362
#3 0x00007fffef92d4e7 in hashmap_base_new.lto_priv () from /lib64/libnss_systemd.so.2
#4 0x00007fffef953f50 in sd_bus_open_system () from /lib64/libnss_systemd.so.2
#5 0x00007fffef943123 in _nss_systemd_getpwuid_r () from /lib64/libnss_systemd.so.2
#6 0x00007ffff707eea5 in getpwuid_r@@GLIBC_2.2.5 () from /lib64/libc.so.6
#7 0x00007ffff707e608 in getpwuid () from /lib64/libc.so.6
#8 0x00005555555859e1 in get_current_user_info.part ()
#9 0x00005555555a375a in initialize_shell_variables ()
#10 0x0000555555580fde in shell_initialize ()
#11 0x00005555555846ff in main ()<Paste>
The reason it fails on my system and not on, for e.g., a new one set up with
mkosi is that the call to getpwuid() depends on the contents
/etc/nsswitch.conf. The systemd module calls syscall() to implement gettid()
which we can't forward due to being a variadic function.
No fix is provided here, but at least it's explained why this happens.
diff --git a/testsuite/init_module.c b/testsuite/init_module.c
index b7d220b..199186b 100644
--- a/testsuite/init_module.c
+++ b/testsuite/init_module.c
@@ -356,9 +356,14 @@
}
/*
- * FIXME: no way to call the libc function - let's hope there are no
- * other users.
+ * FIXME: no way to call the libc function due since this is a
+ * variadic argument function and we don't have a vsyscall() variant
+ * this may fail if a library or process is trying to call syscall()
+ * directly, for example to implement gettid().
*/
+ fprintf(stderr, "FIXME FIXME FIXME: could not wrap call to syscall(%ld), this should not happen\n",
+ __sysno);
+
abort();
}