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From: (Eric W. Biederman)
To: Linus Torvalds <>
Cc: Christoph Hellwig <>,
	Andrew Morton <>,
	Alexander Viro <>,
	Jeremy Kerr <>, Arnd Bergmann <>,
	Oleg Nesterov <>,
	"the arch\/x86 maintainers" <>,
	linuxppc-dev <>,
	linux-fsdevel <>,
	Linux Kernel Mailing List <>
Subject: Re: remove set_fs calls from the coredump code v6
Date: Tue, 05 May 2020 15:28:50 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Linus Torvalds's message of "Tue, 5 May 2020 09:52:04 -0700")

Linus Torvalds <> writes:

> On Tue, May 5, 2020 at 3:13 AM Christoph Hellwig <> wrote:
>> this series gets rid of playing with the address limit in the exec and
>> coredump code.  Most of this was fairly trivial, the biggest changes are
>> those to the spufs coredump code.
> Ack, nice, and looks good.
> The only part I dislike is how we have that 'struct compat_siginfo' on
> the stack, which is a huge waste (most of it is the nasty padding to
> 128 bytes).
> But that's not new, I only reacted to it because the code moved a bit.
> We cleaned up the regular siginfo to not have the padding in the
> kernel (and by "we" I mean "Eric Biederman did it after some prodding
> as part of his siginfo cleanups" - see commit 4ce5f9c9e754 "signal:
> Use a smaller struct siginfo in the kernel"),  and I wonder if we
> could do something similar with that compat thing.
> 128 bytes of wasted kernel stack isn't the end of the world, but it's
> sad when the *actual* data is only 32 bytes or so.

We probably can.   After introducing a kernel_compat_siginfo that is
the size that userspace actually would need.

It isn't something I want to mess with until this code gets merged, as I
think the set_fs cleanups are more important.

Christoph made some good points about how ugly the #ifdefs are in
the generic copy_siginfo_to_user32 implementation.

I am thinking the right fix is to introduce.
	- TS_X32 as a companion to TS_COMPAT in the x86_64.
        - Modify in_x32_syscall() to test TS_X32
        - Implement x32_copy_siginfo_to_user32 that forces TS_X32 to be
          set. AKA:
	        	unsigned long state = current_thread_info()->state;
	                current_thread_info()->state |= TS_X32;
	                current_thread_info()->state = state;

That would make the #ifdefs go away, but I don't yet know what the x86
maintainers would say about that scheme.  I think it is a good path as
it would isolate the runtime cost of that weird SIGCHLD siginfo format
to just x32.  Then ia32 in compat mode would not need to pay.

Once I get that then it will be easier to introduce a yet another helper
of copy_siginfo_to_user32 that generates just the kernel_compat_siginfo
part, and the two visible derivatives can call memset and clear_user
to clear the unset parts.

I am assuming you don't don't mind having a full siginfo in
elf_note_info that ultimately gets copied into the core dump?


  reply	other threads:[~2020-05-05 20:32 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 10:12 remove set_fs calls from the coredump code v6 Christoph Hellwig
2020-05-05 10:12 ` [PATCH 1/7] powerpc/spufs: fix copy_to_user while atomic Christoph Hellwig
2020-05-05 10:12 ` [PATCH 2/7] powerpc/spufs: stop using access_ok Christoph Hellwig
2020-05-05 10:12 ` [PATCH 3/7] powerpc/spufs: simplify spufs core dumping Christoph Hellwig
2020-05-05 10:12 ` [PATCH 4/7] signal: refactor copy_siginfo_to_user32 Christoph Hellwig
2020-05-05 10:12 ` [PATCH 5/7] binfmt_elf: remove the set_fs in fill_siginfo_note Christoph Hellwig
2020-05-05 10:12 ` [PATCH 6/7] binfmt_elf: remove the set_fs(KERNEL_DS) in elf_core_dump Christoph Hellwig
2020-05-05 10:12 ` [PATCH 7/7] binfmt_elf_fdpic: remove the set_fs(KERNEL_DS) in elf_fdpic_core_dump Christoph Hellwig
2020-05-05 16:52 ` remove set_fs calls from the coredump code v6 Linus Torvalds
2020-05-05 20:28   ` Eric W. Biederman [this message]
2020-05-06  6:31     ` Christoph Hellwig
2020-05-06 15:44       ` Eric W. Biederman
2020-05-05 20:34 ` Al Viro
2020-05-05 20:42   ` Christoph Hellwig
2020-05-05 20:47     ` Al Viro

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