lib/syscall_shim: Fix `arch_prctl`'s `ARCH_SET_FS` handling

NOTE: This is interdependent on the `app-elfloader` commit that
changes `arch_prctl`'s `ARCH_SET_FS` behavior, entitled:
```
Do not change real `fs_base` register in `arch_prctl`
```

The code may store and restore userland TLS pointer properly in the
system call handler and it does check if it was changes by syscalls
such as `arch_prctl`'s `ARCH_SET_FS` command. However, one aspect
was missed: `__UK_SYSCALL_RETADDR_CLEAR`. The macro
`__UK_SYSCALL_RETADDR_CLEAR` accesses what is meant to be Unikraft's
TLS and is invoked in the time window between TLS store and restore.
In the `ARCH_SET_FS` case, `arch_prctl` sets the the `fs` register
regardless and thus we end up having the userland desired TLS pointer
during a time window where keeping our own TLS pointer is mandatory.

Thus, to avoid this, do not set any `fs` register value during
`arch_prctl` (see top NOTE) and, instead, simply modify the
`_uk_syscall_ultlsp` value which will be used instead to restore
the userland TLS as the last operation.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>

diff --git a/lib/syscall_shim/uk_syscall_binary.c b/lib/syscall_shim/uk_syscall_binary.c
index f257e88907a64c98fca6ec5dfac0639305e0391c..26b4147a8a711a9abb5b4303ca9caa53a54f6147 100644 (file)
--- a/lib/syscall_shim/uk_syscall_binary.c
+++ b/lib/syscall_shim/uk_syscall_binary.c
@@ -51,7 +51,7 @@ void ukplat_syscall_handler(struct __regs *r)
 {
 #if CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS
        struct uk_thread *self;
-       __uptr orig_tlsp;
+       __uptr ultlsp;
 #endif /* CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS */
        /* Place backup of extended register state on stack */
        __sz ectx_align = ukarch_ectx_align();
@@ -75,11 +75,11 @@ void ukplat_syscall_handler(struct __regs *r)
 
 #if CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS
        /* Activate Unikraft TLS */
-       orig_tlsp = ukplat_tlsp_get();
+       ultlsp = ukplat_tlsp_get();
        self = uk_thread_current();
        UK_ASSERT(self);
        ukplat_tlsp_set(self->uktlsp);
-       _uk_syscall_ultlsp = orig_tlsp;
+       _uk_syscall_ultlsp = ultlsp;
 #endif /* CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS */
 
        /* uk_syscall6_r() will clear _uk_syscall_return_addr on return */
@@ -112,17 +112,20 @@ void ukplat_syscall_handler(struct __regs *r)
 #endif /* CONFIG_LIBSYSCALL_SHIM_STRACE */
 
 #if CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS
-       uk_thread_uktls_var(self, _uk_syscall_ultlsp) = 0x0;
-
        /* Restore original TLS only if it was _NOT_
         * changed by the system call handler
         */
-       if (likely(ukplat_tlsp_get() == self->uktlsp)) {
-               ukplat_tlsp_set(orig_tlsp);
-       } else {
+#if CONFIG_LIBUKDEBUG_PRINTD
+       if (_uk_syscall_ultlsp != ultlsp) {
                uk_pr_debug("System call updated userland TLS pointer register to %p (before: %p)\n",
-                           (void *) orig_tlsp, (void *) ukplat_tlsp_get());
+                           (void *)ultlsp, (void *)ukplat_tlsp_get());
        }
+#endif
+
+       ultlsp = _uk_syscall_ultlsp;
+       uk_thread_uktls_var(self, _uk_syscall_ultlsp) = 0x0;
+
+       ukplat_tlsp_set(ultlsp);
 #endif /* CONFIG_LIBSYSCALL_SHIM_HANDLER_ULTLS */
 
        /* Restore extended register state */