linux-kernel - Re: [PATCH] mm: hwpoison: coredump: support recovery from dump_user

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Message-Id: <20230413150409.f772cbafca5e7e3a658c58ee@linux-foundation.org>
Date:   Thu, 13 Apr 2023 15:04:09 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Naoya Horiguchi <naoya.horiguchi@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        <linux-kernel@...r.kernel.org>, <tongtiangen@...wei.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] mm: hwpoison: coredump: support recovery from
 dump_user_range()

On Thu, 13 Apr 2023 12:13:36 +0800 Kefeng Wang <wangkefeng.wang@...wei.com> wrote:

> The dump_user_range() is used to copy the user page to a coredump
> file, but if a hardware memory error occurred during copy, which
> called from __kernel_write_iter() in dump_user_range(), it crashs,
> 
>  CPU: 112 PID: 7014 Comm: mca-recover Not tainted 6.3.0-rc2 #425
> 
>  pc : __memcpy+0x110/0x260
>  lr : _copy_from_iter+0x3bc/0x4c8
>  ...
>  Call trace:
>   __memcpy+0x110/0x260
>   copy_page_from_iter+0xcc/0x130
>   pipe_write+0x164/0x6d8
>   __kernel_write_iter+0x9c/0x210
>   dump_user_range+0xc8/0x1d8
>   elf_core_dump+0x308/0x368
>   do_coredump+0x2e8/0xa40
>   get_signal+0x59c/0x788
>   do_signal+0x118/0x1f8
>   do_notify_resume+0xf0/0x280
>   el0_da+0x130/0x138
>   el0t_64_sync_handler+0x68/0xc0
>   el0t_64_sync+0x188/0x190
> 
> Generally, the '->write_iter' of file ops will use copy_page_from_iter()
> and copy_page_from_iter_atomic(), change memcpy() to copy_mc_to_kernel()
> in both of them to handle #MC during source read, which stop coredump
> processing and kill the task instead of kernel panic, but the source
> address may not always an user address, so introduce a new copy_mc flag
> in struct iov_iter{} to indicate that the iter could do a safe memory
> copy, also introduce the helpers to set/clear/check the flag, for now,
> it's only used in coredump's dump_user_range(), but it could expand to
> any other scenarios to fix the similar issue.
> 
> ...
>
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -291,6 +291,7 @@ void iov_iter_init(struct iov_iter *i, unsigned int direction,
>  		.nofault = false,
>  		.user_backed = true,
>  		.data_source = direction,
> +		.copy_mc = false,
>  		.__iov = iov,
>  		.nr_segs = nr_segs,
>  		.iov_offset = 0,

hm, linux-next.  mm.git doesn't have the other iov_iter changes, so I
repositioned the iov_iter.copy_mc field so the patch should fuzzily
apply to kernels with or without de4f5fed3f231a8 ("iov_iter: add
iter_iovec() helper"),