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SubjectRe: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v8)
On Wed, 17 Apr 2019, Mathieu Desnoyers wrote:

> > +/* RSEQ_SIG is a signature required before each abort handler code.
> > +
> > + It is a 32-bit value that maps to actual architecture code compiled
> > + into applications and libraries. It needs to be defined for each
> > + architecture. When choosing this value, it needs to be taken into
> > + account that generating invalid instructions may have ill effects on
> > + tools like objdump, and may also have impact on the CPU speculative
> > + execution efficiency in some cases. */
> > +
> > +#define RSEQ_SIG 0xd428bc00 /* BRK #0x45E0. */
>
> After further investigation, we should probably do the following
> to handle compiling with -mbig-endian on aarch64, which generates
> binaries with mixed code vs data endianness (little endian code,
> big endian data):

First, the comment on RSEQ_SIG should specify whether it is to be
interpreted in the code or the data endianness.

> For ARM32, the situation is a bit more complex. Only armv6+
> generates mixed-endianness code vs data with -mbig-endian.
> Prior to armv6, the code and data endianness matches. Therefore,
> I plan to #ifdef the reversed endianness handling with:
>
> #if __ARM_ARCH >= 6 && __ARM_BIG_ENDIAN
>
> on arm32.

That doesn't work well because BE code (.o files) can be built for v5te
(for example) and used on a range of different architecture variants with
both BE32 and BE8 - the choice between BE32 and BE8 is a link-time choice,
not a compile-time choice. So if the value for Arm is a compile-time
constant, it should also work for both BE32 and BE8.

In turn, that suggests to me that RSEQ_SIG should be defined to be a value
that is always in the code endianness (and whatever corresponding kernel
code handles RSEQ_SIG values should act accordingly on architectures where
the two endiannesses can differ). If the kernel ABI is already fixed in a
way that prevents such a definition of RSEQ_SIG semantics as using code
endianness, a value should be chosen for Arm that works for both
endiannesses.

(Also, installed glibc headers are supposed to work with older compilers,
and support for __ARM_ARCH was only added in GCC 4.8. Before that you
need to test lots of separate macros for different architecture variants
to determine a version number.)

--
Joseph S. Myers
joseph@codesourcery.com

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