linux-user/syscall.c: Don't warn about unimplemented get_robust_list

The nature of the kernel ABI for the get_robust_list and set_robust_list
syscalls means we cannot implement them in QEMU. Make get_robust_list
silently return ENOSYS rather than using the default "print message and
then fail ENOSYS" code path, in the same way we already do for
set_robust_list, and add a comment documenting why we do this.

This silences warnings which were being produced for emulating
even trivial programs like 'ls' in x86-64-on-x86-64.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/syscall.c

@@ -8631,6 +8631,19 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 
 #ifdef TARGET_NR_set_robust_list
     case TARGET_NR_set_robust_list:
+    case TARGET_NR_get_robust_list:
+        /* The ABI for supporting robust futexes has userspace pass
+         * the kernel a pointer to a linked list which is updated by
+         * userspace after the syscall; the list is walked by the kernel
+         * when the thread exits. Since the linked list in QEMU guest
+         * memory isn't a valid linked list for the host and we have
+         * no way to reliably intercept the thread-death event, we can't
+         * support these. Silently return ENOSYS so that guest userspace
+         * falls back to a non-robust futex implementation (which should
+         * be OK except in the corner case of the guest crashing while
+         * holding a mutex that is shared with another process via
+         * shared memory).
+         */
         goto unimplemented_nowarn;
 #endif