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Date: Tue, 12 Nov 2019 11:34:46 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH 0/6] updates for linux v5.3
On Tue, Nov 12, 2019 at 12:16:06PM +0100, Szabolcs Nagy wrote:
> * Rich Felker <dalias@...c.org> [2019-11-11 22:01:50 -0500]:
>
> > On Sun, Nov 10, 2019 at 01:02:53PM +0100, Szabolcs Nagy wrote:
> > > * Rich Felker <dalias@...c.org> [2019-11-10 01:14:09 -0500]:
> > > > On Sun, Nov 10, 2019 at 02:08:02AM +0100, Szabolcs Nagy wrote:
> > > > > >From 560fd1ebe616fd59c0abcaf86bec6109bfcd2141 Mon Sep 17 00:00:00 2001
> > > > > From: Szabolcs Nagy <nsz@...t70.net>
> > > > > Date: Sun, 3 Nov 2019 22:45:05 +0000
> > > > > Subject: [PATCH 4/6] sys/ptrace.h: add PTRACE_GET_SYSCALL_INFO from linux v5.3
> > > > >
> > > > > ptrace API to get details of the syscall the tracee is blocked in, see
> > > > >
> > > > > linux commit 201766a20e30f982ccfe36bebfad9602c3ff574a
> > > > > ptrace: add PTRACE_GET_SYSCALL_INFO request
> > > > >
> > > > > the align attribute was used to keep the layout the same across targets
> > > > > e.g. on m68k uint32_t is 2 byte aligned, this helps with compat ptrace.
> > > >
> > > > Can you explain the motivation for this? At first I thought it was for
> > > > overall alignment of the structure, but there are also 64-bit members
> > > > that aren't aligned, so presumably this is only to get padding after
> > > > the initial uint8_t? If so, just add 3 explicit padding members:
> > >
> > > the original linux struct had padding but during
> > > review they changed it to aligned because some
> > > linux devs thought that made the intent clearer
> > > or more future proof (e.g. what if uint64_t is
> > > also 2byte aligned, but on the 64bit version of
> > > the same architecture it's 8byte aligned, then
> > > compat ptrace would not work because one abi
> > > would have padding and the other wouldnt).
> >
> > I don't follow that line of reasoning; the alignment would potentially
> > differ, but the layout wouldn't, and that's why I initially suspected
> > they were doing this for alignment. In any case, the only arch without
> > at least 4-byte alignment is m68k, and it's not going to have a 64-bit
> > version.
>
> why would the layout be the same?
>
> uint8_t x;
> uint64_t y; // aligned to 2 bytes
>
> and
>
> uint8_t x;
> uint64_t y; // aligned to 4 bytes
>
> should have different layout (1 vs 3 bytes padding).
I'm talking about doing it with explicit padding, and thought that's
what we were comparing against, e.g.:
uint8_t x, __pad[7];
uint64_t y;
> > > i guess for musl either works, but the current
> > > struct is how it is defined in glibc.
> >
> > Generally in musl we prefer not using extensions in public headers
> > except where the effect can be achieved in no other way. Only m68k is
> > affected by using the forced alignment here, but I don't think there's
> > any strong reason to prefer one way or the other. I think I'd want to
> > include the padding even if we do also include
> > _Alignas/alignas/__attribute__((__aligned__)) (dependent on language
> > version macros) so that in the fallback case the layout is still
> > correct even if the alignment isn't (on m68k only).
>
> ok with me.
Any preference on which (also having the aligned, or not)? I'd
probably lean towards omitting it but I don't have a strong opinion on
this.
Rich
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