A little patch fix dev_read to make it works with large file

[Gao Xiang]

Hi LinXuan,

On Thu, Oct 20, 2022 at 04:12:57PM +0800, chenlinxuan wrote:
> When using `fsck.erofs` to extract some image have a very large file (3G)
> inside, my fsck.erofs report some thing like:
> 
> <E> erofs_io: Failed to read data from device - erofs.image:[4096,
> 2147483648].
> <E> erofs: failed to read data of m_pa 4096, m_plen 2147483648 @ nid 40: -17
> <E> erofs: Failed to extract filesystem
> 
> You can use this script to reproduce this issue.
> 
> #!/bin/env sh
> 
> mkdir tmp extract
> dd if=/dev/urandom of=tmp/big_file bs=1M count=2048
> 
> mkfs.erofs erofs.image tmp
> fsck.erofs erofs.image --extract=extract
> 
> I found that dev_open will failed if we can not get all data we want with
> one pread call.
> 
> I write this little patch try to fix this issue.
> 
> This is my first patch send via email, sorry if anything goes wrong.
> 

Thanks for your report, and the issue looks true.

In order to merge this patch, could you apply the message above into
the commit message?

BTW, the commit message can be updated with "git commit --amend".

> From 156af5b173c1f9e2a91e4d2126214b96966babd1 Mon Sep 17 00:00:00 2001
> From: chenlinxuan <chenlinxuan at uniontech.com>

It would be better to update your real name into
"Linxuan Chen" Or "Chen Linxuan" (whichever you prefer).

> Date: Thu, 20 Oct 2022 14:39:17 +0800
> Subject: [PATCH] erofs-utils: lib: fix dev_read

Subject title:
"erofs-utils: lib: fix dev_read to make it works with large file"

> 
> We need to keep calling pread until we get all data we want
> ---
>  lib/io.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/io.c b/lib/io.c
> index 524cfb4..bd3d790 100644
> --- a/lib/io.c
> +++ b/lib/io.c
> @@ -256,7 +256,7 @@ int dev_resize(unsigned int blocks)
> 
>  int dev_read(int device_id, void *buf, u64 offset, size_t len)
>  {
> -   int ret, fd;
> +   int read_count, fd;
> 
>     if (cfg.c_dry_run)
>         return 0;
> @@ -278,15 +278,29 @@ int dev_read(int device_id, void *buf, u64 offset,
> size_t len)
>         fd = erofs_blobfd[device_id - 1];
>     }
> 
> +   while (len > 0) {
>  #ifdef HAVE_PREAD64
> -   ret = pread64(fd, buf, len, (off64_t)offset);
> +       read_count = pread64(fd, buf, len, (off64_t)offset);
>  #else
> -   ret = pread(fd, buf, len, (off_t)offset);
> +       read_count = pread(fd, buf, len, (off_t)offset);
>  #endif
> -   if (ret != (int)len) {
> -       erofs_err("Failed to read data from device - %s:[%" PRIu64 ",
> %zd].",
> -             erofs_devname, offset, len);
> -       return -errno;
> +       if (read_count == -1 || read_count ==  0) {

                                                ^ extra space here.

Thanks,
Gao Xiang