linux-kernel - Re: [PATCH] Revert "bcachefs: Add asserts to bch2_dev_btree_bitmap_marked

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Message-ID: <rd4maufxfunrcpzkoo2jszorzl256fiktw3p5yfpnjpu2ajh4l@5wvkfxwzajax>
Date: Thu, 24 Oct 2024 07:36:46 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: manas18244@...td.ac.in
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Anup Sharma <anupnewsmail@...il.com>, Shuah Khan <skhan@...uxfoundation.org>, 
	syzbot+e8eff054face85d7ea41@...kaller.appspotmail.com
Subject: Re: [PATCH] Revert "bcachefs: Add asserts to
 bch2_dev_btree_bitmap_marked_sectors()"

On Mon, Oct 21, 2024 at 10:18:57PM +0530, Manas via B4 Relay wrote:
> From: Manas <manas18244@...td.ac.in>
> 
> This reverts commit 60f2b1bcf519416dbffee219132aa949d0c39d0e.
> 
> This syzbot bug[1] is triggered due to the BUG_ON assertions added in
> __bch2_dev_btree_bitmap_mark. During runtime, m->btree_bitmap_shift is
> 63 '?'. This triggers both the assertions.

The BUG_ON() doesn't need to be deleted; we just need to fix the
validation so it doesn't fire (it doesn't particularly matter if it's
removed or not, ubsan would catch it without the BUG_ON()).

I believe 57 is correct, 64 - (2^6, which is 64, i.e. size of the btree
bitmap), but >= 58 would have been better.

> 
> Reverting the commit does not reproduce the said bug.
> 
> [1] https://syzkaller.appspot.com/bug?extid=e8eff054face85d7ea41
> 
> Signed-off-by: Manas <manas18244@...td.ac.in>
> Reported-by: syzbot+e8eff054face85d7ea41@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=e8eff054face85d7ea41
> ---
> This syzbot bug[1] is triggered due to the BUG_ON assertions added in
> __bch2_dev_btree_bitmap_mark. During runtime, m->btree_bitmap_shift is
> 63 '?'. This triggers both the assertions.
> 
> I am unfamiliar with the codebase, and there wasn't a lore discussion
> about the assertions in the commit, so I am unsure about the relevance
> of these assertions.
> 
> Reverting the commit does not reproduce the said bug.
> 
> [1] https://syzkaller.appspot.com/bug?extid=e8eff054face85d7ea41
> ---
>  fs/bcachefs/sb-members.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/fs/bcachefs/sb-members.c b/fs/bcachefs/sb-members.c
> index fb08dd680dacf82bca414f424024e4a00bf432de..9790fd47338c46d2af30547e1f41a1e578b71aa4 100644
> --- a/fs/bcachefs/sb-members.c
> +++ b/fs/bcachefs/sb-members.c
> @@ -450,9 +450,6 @@ static void __bch2_dev_btree_bitmap_mark(struct bch_sb_field_members_v2 *mi, uns
>  		m->btree_bitmap_shift += resize;
>  	}
>  
> -	BUG_ON(m->btree_bitmap_shift > 57);
> -	BUG_ON(end > 64ULL << m->btree_bitmap_shift);
> -
>  	for (unsigned bit = start >> m->btree_bitmap_shift;
>  	     (u64) bit << m->btree_bitmap_shift < end;
>  	     bit++)
> 
> ---
> base-commit: 42f7652d3eb527d03665b09edac47f85fb600924
> change-id: 20241021-revert-assert-bch2-25474fe809d1
> 
> Best regards,
> -- 
> Manas <manas18244@...td.ac.in>
> 
>