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Date: Wed, 25 May 2022 08:32:12 -0400
From: Rich Felker <dalias@...c.org>
To: 王洪亮 <wanghongliang@...ngson.cn>
Cc: musl@...ts.openwall.com
Subject: Re: add loongarch64 port v3
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
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