linux-kernel - Re: BISECT result: 6.0.0-RC kernels trigger Firefox snap bug with 6.0.0-rc3 through 6.0.0-rc7

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Message-ID: <0df684f9-b3c2-3511-fa12-b38fe5fee1a5@squashfs.org.uk>
Date:   Thu, 20 Oct 2022 16:49:21 +0100
From:   Phillip Lougher <phillip@...ashfs.org.uk>
To:     Jintao Yin <nicememory@...il.com>
Cc:     bagasdotme@...il.com, hsinyi@...omium.org,
        linux-kernel@...r.kernel.org, marcmiltenberger@...il.com,
        mirsad.todorovac@....unizg.hr, regressions@...mhuis.info,
        regressions@...ts.linux.dev, srw@...dewatkins.net
Subject: Re: BISECT result: 6.0.0-RC kernels trigger Firefox snap bug with
 6.0.0-rc3 through 6.0.0-rc7

On 20/10/2022 14:55, Jintao Yin wrote:
> Hi all,
> 
> After review the details of page actor, the tail bytes may be written to
> a temp buffer instead the last used page. So before diff would wrongly
> memzero a page which is not the tail bytes in.

If you *only* discovered that *after* writing the patch, it is clear
you don't fully understand what is going on.

Cheers

Phillip

> 
> In this diff fixes it by caculation of the real index the trailing bytes
> in and check if the last used page matches this index. If the page is
> the real tail bytes in, then memzero the trailing bypte of the page.
> 
> Please help test and any feedbacks are welcome.
> 
> Thanks,
> 
> Jintao
> 
> 
> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> index e56510964b22..e1fafd10a850 100644
> --- a/fs/squashfs/file.c
> +++ b/fs/squashfs/file.c
> @@ -506,8 +506,9 @@ static int squashfs_readahead_fragment(struct page **page,
>   		squashfs_i(inode)->fragment_size);
>   	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
>   	unsigned int n, mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
> +	int res = buffer->error;
>   
> -	if (buffer->error)
> +	if (res)
>   		goto out;
>   
>   	expected += squashfs_i(inode)->fragment_offset;
> @@ -529,7 +530,7 @@ static int squashfs_readahead_fragment(struct page **page,
>   
>   out:
>   	squashfs_cache_put(buffer);
> -	return buffer->error;
> +	return res;
>   }
>   
>   static void squashfs_readahead(struct readahead_control *ractl)
> @@ -557,6 +558,7 @@ static void squashfs_readahead(struct readahead_control *ractl)
>   		int res, bsize;
>   		u64 block = 0;
>   		unsigned int expected;
> +		int nr_used_pages;
>   
>   		nr_pages = __readahead_batch(ractl, pages, max_pages);
>   		if (!nr_pages)
> @@ -593,18 +595,21 @@ static void squashfs_readahead(struct readahead_control *ractl)
>   
>   		res = squashfs_read_data(inode->i_sb, block, bsize, NULL, actor);
>   
> -		squashfs_page_actor_free(actor);
> +		nr_used_pages = squashfs_page_actor_free(actor);
>   
>   		if (res == expected) {
>   			int bytes;
> +			pgoff_t bytes_index;
>   
>   			/* Last page (if present) may have trailing bytes not filled */
>   			bytes = res % PAGE_SIZE;
> -			if (pages[nr_pages - 1]->index == file_end && bytes)
> -				memzero_page(pages[nr_pages - 1], bytes,
> +			bytes_index = (index << shift) + ((res - bytes) >> PAGE_SHIFT);
> +			if (bytes && nr_used_pages > 0 &&
> +				pages[nr_used_pages - 1]->index == bytes_index)
> +				memzero_page(pages[nr_used_pages - 1], bytes,
>   					     PAGE_SIZE - bytes);
>   
> -			for (i = 0; i < nr_pages; i++) {
> +			for (i = 0; i < nr_used_pages; i++) {
>   				flush_dcache_page(pages[i]);
>   				SetPageUptodate(pages[i]);
>   			}
> diff --git a/fs/squashfs/file_direct.c b/fs/squashfs/file_direct.c
> index f1ccad519e28..ee462ef380bf 100644
> --- a/fs/squashfs/file_direct.c
> +++ b/fs/squashfs/file_direct.c
> @@ -26,14 +26,14 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
>   	struct inode *inode = target_page->mapping->host;
>   	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
>   
> -	int file_end = (i_size_read(inode) - 1) >> PAGE_SHIFT;
> -	int mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
> -	int start_index = target_page->index & ~mask;
> -	int end_index = start_index | mask;
> -	int i, n, pages, bytes, res = -ENOMEM;
> +	pgoff_t file_end = (i_size_read(inode) - 1) >> PAGE_SHIFT;
> +	pgoff_t mask = (1 << (msblk->block_log - PAGE_SHIFT)) - 1;
> +	pgoff_t start_index = target_page->index & ~mask;
> +	pgoff_t end_index = start_index | mask;
> +	int i, pages, used_pages, bytes, res = -ENOMEM;
> +	pgoff_t n, bytes_index;
>   	struct page **page;
>   	struct squashfs_page_actor *actor;
> -	void *pageaddr;
>   
>   	if (end_index > file_end)
>   		end_index = file_end;
> @@ -74,7 +74,7 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
>   	/* Decompress directly into the page cache buffers */
>   	res = squashfs_read_data(inode->i_sb, block, bsize, NULL, actor);
>   
> -	squashfs_page_actor_free(actor);
> +	used_pages = squashfs_page_actor_free(actor);
>   
>   	if (res < 0)
>   		goto mark_errored;
> @@ -86,16 +86,19 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
>   
>   	/* Last page (if present) may have trailing bytes not filled */
>   	bytes = res % PAGE_SIZE;
> -	if (page[pages - 1]->index == end_index && bytes) {
> -		pageaddr = kmap_local_page(page[pages - 1]);
> -		memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
> -		kunmap_local(pageaddr);
> +	bytes_index = start_index + ((res - bytes) >> PAGE_SHIFT);
> +	if (used_pages > 0 && bytes &&
> +		page[used_pages - 1]->index == bytes_index) {
> +		memzero_page(page[used_pages - 1], bytes,
> +			     PAGE_SIZE - bytes);
>   	}
>   
>   	/* Mark pages as uptodate, unlock and release */
>   	for (i = 0; i < pages; i++) {
> -		flush_dcache_page(page[i]);
> -		SetPageUptodate(page[i]);
> +		if (i < used_pages) {
> +			flush_dcache_page(page[i]);
> +			SetPageUptodate(page[i]);
> +		}
>   		unlock_page(page[i]);
>   		if (page[i] != target_page)
>   			put_page(page[i]);
> @@ -112,8 +115,10 @@ int squashfs_readpage_block(struct page *target_page, u64 block, int bsize,
>   	for (i = 0; i < pages; i++) {
>   		if (page[i] == NULL || page[i] == target_page)
>   			continue;
> -		flush_dcache_page(page[i]);
> -		SetPageError(page[i]);
> +		if (i < used_pages) {
> +			flush_dcache_page(page[i]);
> +			SetPageError(page[i]);
> +		}
>   		unlock_page(page[i]);
>   		put_page(page[i]);
>   	}
> diff --git a/fs/squashfs/page_actor.h b/fs/squashfs/page_actor.h
> index 95ffbb543d91..c2c5c3937ef9 100644
> --- a/fs/squashfs/page_actor.h
> +++ b/fs/squashfs/page_actor.h
> @@ -29,10 +29,12 @@ extern struct squashfs_page_actor *squashfs_page_actor_init(void **buffer,
>   extern struct squashfs_page_actor *squashfs_page_actor_init_special(
>   				struct squashfs_sb_info *msblk,
>   				struct page **page, int pages, int length);
> -static inline void squashfs_page_actor_free(struct squashfs_page_actor *actor)
> +static inline int squashfs_page_actor_free(struct squashfs_page_actor *actor)
>   {
> +	int res = actor->next_page;
>   	kfree(actor->tmp_buffer);
>   	kfree(actor);
> +	return res;
>   }
>   static inline void *squashfs_first_page(struct squashfs_page_actor *actor)
>   {