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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>
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1 parent a94b498 commit e9a970a8316f9f86a6c800a9a90175bd593f862c @pm215 pm215 committed with suihkulokki Feb 8, 2013
Showing with 14 additions and 1 deletion.
  1. +14 −1 linux-user/syscall.c
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@@ -8631,7 +8631,20 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
#ifdef TARGET_NR_set_robust_list
case TARGET_NR_set_robust_list:
- goto unimplemented_nowarn;
+ 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
#if defined(TARGET_NR_utimensat) && defined(__NR_utimensat)

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