linux-kernel - Re: [PATCH 0/3] arm64/sve: UAPI: Disentangle ptrace.h from sigcontext.h

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Message-ID: <5e96f169-d77d-63d0-11ba-f59f572b0219@arm.com>
Date:   Fri, 14 Dec 2018 19:00:07 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Dave P Martin <Dave.Martin@....com>
CC:     nd <nd@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Will Deacon <Will.Deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alan Hayward <Alan.Hayward@....com>
Subject: Re: [PATCH 0/3] arm64/sve: UAPI: Disentangle ptrace.h from
 sigcontext.h

On 14/12/2018 18:25, Dave Martin wrote:
> On Fri, Dec 14, 2018 at 06:13:33PM +0000, Szabolcs Nagy wrote:
>> On 11/12/2018 19:26, Dave Martin wrote:
>>> This patch refactors the UAPI header definitions for the Arm SVE
>>> extension to avoid multiple-definition problems when userspace mixes its
>>> own sigcontext.h definitions with the kernel's ptrace.h (which is
>>> apparently routine).
>>>
>>> A common backend header is created to hold common definitions, suitably
>>> namespaced, and with an appropriate header guard.
>>>
>>> See the commit message in patch 3 for further explanation of why this
>>> is needed.
>>>
>>> Because of the non-trivial header guard in the new sve_context.h, patch
>>> 1 adds support to headers_install.sh to munge #if defined(_UAPI_FOO) in
>>> a similar way to the current handling of #ifndef _UAPI_FOO.
>>>
>>
>> thanks for doing this.
>>
>> the patches fix the gdb build issue on musl libc with an
>> additional gdb patch:
>> https://sourceware.org/ml/gdb-patches/2018-12/msg00152.html
>> (in userspace i'd expect users relying on signal.h providing
>> whatever is in asm/sigcontext.h.)
>>
>> i think sve_context.h could be made to work with direct include,
>> even if that's not useful because there is no public api there.
>> (and then you dont need the first patch)
> 
> My general view is that if you want the sigframe types userspace should
> usually include <ucontext.h> and refer to mcontext_t.
> 

ucontext.h does not expose the asm/sigcontext.h types in glibc,
but it is compatible with the inclusion of asm/sigcontext.h
(or signal.h).

in musl ucontext.h includes signal.h and signal.h provides
the asm/sigcontext.h api with abi compatible definitions.

> Because the prototype for sa_sigaction() specifies a void * for the
> ucontext argument, I've generally assumed that <signal.h> is not
> sufficient to get ucontext_t (or mcontext_t) (but maybe I'm too paranoid
> there).

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html

"The <signal.h> header shall define the ucontext_t type as a structure
 that shall include at least the following members:
...
 mcontext_t  uc_mcontext A machine-specific representation of the saved
             context."

so signal.h must define ucontext_t but mcontext_t can be opaque.
(it is opaque with posix conform feature tests to avoid
namespace pollution, but with _GNU_SOURCE defined all
asm/sigcontext.h apis are there and mcontext_t matches
struct sigcontext)

> 
> Non-POSIX-flavoured software might include <asm/sigcontext.h> directly.
> In glibc/musl libc will that conflict with <signal.h>, or can the two
> coexist?

if you compile e.g in standard conform mode then
i think signal.h and asm/sigcontext.h are compatible.

> 
> Cheers
> ---Dave
>