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Date: Wed, 30 Mar 2022 08:57:55 +0200
From: Harald Hoyer <>
To: Rich Felker <>
Subject: Re: [PATCH 1/1] feat(x86_64): use wrfsbase if AT_HWCAP2 allows

Am 29.03.22 um 17:54 schrieb Rich Felker:
> On Tue, Mar 29, 2022 at 02:24:16PM +0200, Harald Hoyer wrote:
>> If `AT_HWCAP2` has `HWCAP2_FSGSBASE` set, then instead of calling
>> `arch_prctl()`, the `wrfsbase` instruction will be used.
>> This is helpful in SGX contexts, where inside the enclave no other
>> mechanism is possible.
> Thanks for including this motivation, since otherwise I don't think it
> makes any sense to use this feature. BTW what happens with other
> syscalls in such a context (at least set_tid_address is called
> unconditionally), and how does the process communicate any information
> or even exit?

In our project (enarx) we catch the `syscall` exception and handle it by proxying
it to the host (with filtering and sanity checks). Because `arch_prctl` sets a
hardware register, which is handled special in enclave context switching,
we can't do that for this syscall.

>> diff --git a/src/thread/x86_64/__set_thread_area.c b/src/thread/x86_64/__set_thread_area.c
>> new file mode 100644
>> index 00000000..dcc5d116
>> --- /dev/null
>> +++ b/src/thread/x86_64/__set_thread_area.c
>> @@ -0,0 +1,14 @@
>> +#include <libc.h>
>> +#include <syscall.h>
>> +#include <bits/hwcap.h>
>> +
>> +hidden int __set_thread_area(void *p)
>> +{
>> +	if (__hwcap2 & HWCAP2_FSGSBASE) {
>> +		__asm__ ("wrfsbase %0" :: "r" (p) : "memory");
>> +		return 0;
>> +	}
>> +
>> +	// arch_prctl(SET_FS, arg)
>> +	return syscall(__NR_arch_prctl, 0x1002, p);
>> +}
> I'm guessing this breaks build on anything but recent assembler
> versions, no? If so, it should probably be written with a .byte
> directive or something.

Is gcc-4.6.0 old enough?

commit 4ee89d5fb78b48b62b507a29d3a576c63ae22505
Author: H.J. Lu <>
Date:   Mon Jul 5 21:57:55 2010 +0000

or llvm 3.1.0?

commit 228d9131aad9f3553c9c69abf010f41ce43c8423
Author: Craig Topper <>
Date:   Sun Oct 30 19:57:21 2011 +0000

> There's also a question of whether the existence of the hwcap flag is
> intended to document a contract for the kernel to permit the process
> to perform this operation, and a contract for the kernel to accept it
> being set this way (rather than creating a possible inconsistency
> between the kernel's idea of the process's %fs base and the actual %fs
> base that's active. Is this documented somewhere on the kernel side?
> If so then this should be okay, but this needs checking before it can
> be merged.
> Rich

linux Documentation/x86/x86_64/fsgs.rst:

  The kernel provides reliable information about the enabled state in the
  ELF AUX vector. If the HWCAP2_FSGSBASE bit is set in the AUX vector, the
  kernel has FSGSBASE instructions enabled and applications can use them.

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