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From: Rich Felker <dalias@libc.org>
To: 王洪亮 <wanghongliang@loongson.cn>
Cc: musl@lists.openwall.com
Subject: Re: [musl] add loongarch64 port v3
Date: Wed, 25 May 2022 08:32:12 -0400	[thread overview]
Message-ID: <20220525123211.GV7074@brightrain.aerifal.cx> (raw)
In-Reply-To: <4978b83d-871a-e8ca-ab4b-6dec73f34075@loongson.cn>

On Wed, May 25, 2022 at 06:08:23PM +0800, 王洪亮 wrote:
> 
> 在 2022/5/24 下午8:32, Rich Felker 写道:
> >What we've been trying to say is that there are several cases, none of
> >which seem to need it:
> >
> >1. You create an object with declared type struct sigcontext. In this
> >    case, the flexible array member at the end is not present at all
> >    (because that's how C works) which means there's no extended
> >    context which needs additional alignment and probably also means
> >    this is not a usable way of creating sigcontext structs.
> >
> >2. You malloc storage for the object with space for the flexible array
> >    member. In this case the allocation has alignment max_align_t and
> >    everything is fine.
> >
> >3. You get the object from the kernel pushing it onto the stack in a
> >    signal frame. This is probably actually the only case the type is
> >    usable in, and of course it has whatever alignment the kernel gave
> >    it.
> Specify the __attribute__((__aligned__(16))) in musl,is used to be
> consistent with kernel.if I removed the __attribute__((__aligned__(16))),
> there is a libc-test fail in pthread_cancel.exe.the reason is that the
> offset of uc->uc_mcontext from the start of uc obtained in cancel_handler
> is inconsistent with kernel pushing it onto the stack in a signal frame.
> so I understand the __attribute__((__aligned__(16))) is necessary in musl.
> 
> src/thread/pthread_cancel.c
> 
> static void cancel_handler(int sig, siginfo_t *si, void *ctx)
> {
>         pthread_t self = __pthread_self();
>         ucontext_t *uc = ctx;
>         uintptr_t pc = uc->uc_mcontext.MC_PC;
>          ...
> }
> 
> musl/arch/loongarch64/bits/signal.h:
> 
> typedef unsigned long greg_t, gregset_t[32];
> typedef struct sigcontext {
>         unsigned long pc;
>         gregset_t gregs;
>         unsigned int flags;
>         unsigned long extcontext[];
> }mcontext_t;
> 
> linux/arch/loongarch/include/uapi/asm/sigcontext.h:
> 
> struct sigcontext {
>         __u64   sc_pc;
>         __u64   sc_regs[32];
>         __u32   sc_flags;
>         __u64   sc_extcontext[0] __attribute__((__aligned__(16)));
> };

This is because ucontext_t is defined without explicit padding before
uc_mcontext. Add "long __uc_pad;" or similar before it so that the
offset is explicitly what it's supposed to be rather than a
consequence ot overalignment.

Rich

  reply	other threads:[~2022-05-25 12:32 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-05  3:21 王洪亮
2022-05-09  1:34 ` [musl] " 王洪亮
2022-05-11 13:37   ` Rich Felker
2022-05-11 16:08     ` Arnd Bergmann
2022-05-16 14:27 ` [musl] " Rich Felker
2022-05-21  6:38   ` 王洪亮
2022-05-21  8:22     ` Markus Wichmann
2022-05-24  9:08       ` 王洪亮
2022-05-24 12:32         ` Rich Felker
2022-05-25 10:08           ` 王洪亮
2022-05-25 12:32             ` Rich Felker [this message]
2022-05-26  3:07               ` 王洪亮
2022-05-29  6:34                 ` Szabolcs Nagy

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