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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Geoff Levand <geoff@infradead.org>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Arnd Bergmann <arnd@arndb.de>
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        <20201123175052.GA20279@redhat.com>
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Date:   Wed, 02 Dec 2020 09:20:08 -0600
In-Reply-To: <87h7pdnlzv.fsf_-_@x220.int.ebiederm.org> (Eric W. Biederman's
        message of "Wed, 25 Nov 2020 15:51:32 -0600")
Message-ID: <87sg8ock0n.fsf@x220.int.ebiederm.org>
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Subject: Re: [RFC][PATCH] coredump: Document coredump code exclusively used by cell spufs
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ebiederm@xmission.com (Eric W. Biederman) writes:

> Oleg Nesterov recently asked[1] why is there an unshare_files in
> do_coredump.  After digging through all of the callers of lookup_fd it
> turns out that it is
> arch/powerpc/platforms/cell/spufs/coredump.c:coredump_next_context
> that needs the unshare_files in do_coredump.
>
> Looking at the history[2] this code was also the only piece of coredump code
> that required the unshare_files when the unshare_files was added.
>
> Looking at that code it turns out that cell is also the only
> architecture that implements elf_coredump_extra_notes_size and
> elf_coredump_extra_notes_write.
>
> I looked at the gdb repo[3] support for cell has been removed[4] in binutils
> 2.34.  Geoff Levand reports he is still getting questions on how to
> run modern kernels on the PS3, from people using 3rd party firmware so
> this code is not dead.  According to Wikipedia the last PS3 shipped in
> Japan sometime in 2017.  So it will probably be a little while before
> everyone's hardware dies.
>
> Add some comments briefly documenting the coredump code that exists
> only to support cell spufs to make it easier to understand the
> coredump code.  Eventually the hardware will be dead, or their won't
> be userspace tools, or the coredump code will be refactored and it
> will be too difficult to update a dead architecture and these comments
> make it easy to tell where to pull to remove cell spufs support.
>
> [1] https://lkml.kernel.org/r/20201123175052.GA20279@redhat.com
> [2] 179e037fc137 ("do_coredump(): make sure that descriptor table isn't shared")
> [3] git://sourceware.org/git/binutils-gdb.git
> [4] abf516c6931a ("Remove Cell Broadband Engine debugging support").
> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
>
> Does this change look good to people?  I think it captures this state of
> things and makes things clearer without breaking anything or removing
> functionality for anyone.

I haven't heard anything except a general ack to the concept of
comments.  So I am applying this.

Eric

>
>  fs/binfmt_elf.c | 2 ++
>  fs/coredump.c   | 1 +
>  2 files changed, 3 insertions(+)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index b6b3d052ca86..c1996f0aeaed 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -2198,6 +2198,7 @@ static int elf_core_dump(struct coredump_params *cprm)
>  	{
>  		size_t sz = get_note_info_size(&info);
>  
> +		/* For cell spufs */
>  		sz += elf_coredump_extra_notes_size();
>  
>  		phdr4note = kmalloc(sizeof(*phdr4note), GFP_KERNEL);
> @@ -2261,6 +2262,7 @@ static int elf_core_dump(struct coredump_params *cprm)
>  	if (!write_note_info(&info, cprm))
>  		goto end_coredump;
>  
> +	/* For cell spufs */
>  	if (elf_coredump_extra_notes_write(cprm))
>  		goto end_coredump;
>  
> diff --git a/fs/coredump.c b/fs/coredump.c
> index abf807235262..3ff17eea812e 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -790,6 +790,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>  	}
>  
>  	/* get us an unshared descriptor table; almost always a no-op */
> +	/* The cell spufs coredump code reads the file descriptor tables */
>  	retval = unshare_files();
>  	if (retval)
>  		goto close_fail;