Re: [PATCH 0/6] updates for linux v5.3

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2019-11-11 22:01:50 -0500]:

> On Sun, Nov 10, 2019 at 01:02:53PM +0100, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@libc.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@port70.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 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.