[PATCH] fs: Allow number of blocks in block list to be optional, defaulting length to device length

[PATCH] fs: Allow number of blocks in block list to be optional, defaulting length to device length

[Glenn Washburn]

This is primarily useful to do something like "loopback newdev (dev)8+" to
create a device that skips the first 4K, which may contain a container
header, eg. a non-standard RAID1 header, that grub does not recognize. This
would allow that container data to be potentially accessed up to the end of
container, which may be necessary for some layouts that store data at the
end. There is currently not a good way to programmatically get the number
of sectors on a disk to set the appropriate length of the blocklist.

Signed-off-by: Glenn Washburn <development@efficientek.com>
---
 docs/grub.texi      | 10 ++++++----
 grub-core/kern/fs.c |  6 +++++-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/docs/grub.texi b/docs/grub.texi
index eeb3118eb..b1abb6c6a 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -3036,16 +3036,18 @@ snapshot name is omitted.
 
 A block list is used for specifying a file that doesn't appear in the
 filesystem, like a chainloader. The syntax is
-@code{[@var{offset}]+@var{length}[,[@var{offset}]+@var{length}]@dots{}}.
+@code{[@var{offset}]+[@var{length}][,[@var{offset}]+[@var{length}]]@dots{}}.
 Here is an example:
 
 @example
-@code{0+100,200+1,300+300}
+@code{0+100,200+1,300+300,800+}
 @end example
 
 This represents that GRUB should read blocks 0 through 99, block 200,
-and blocks 300 through 599. If you omit an offset, then GRUB assumes
-the offset is zero.
+blocks 300 through 599, and blocks 800 until the end of the device.
+If you omit an offset, then GRUB assumes the offset is zero. If the
+length is omitted, then GRUB assumes the block list extends until the
+end of the device.
 
 Like the file name syntax (@pxref{File name syntax}), if a blocklist
 does not contain a device name, then GRUB uses GRUB's @dfn{root
diff --git a/grub-core/kern/fs.c b/grub-core/kern/fs.c
index c698295bc..e0d7e16a2 100644
--- a/grub-core/kern/fs.c
+++ b/grub-core/kern/fs.c
@@ -173,7 +173,11 @@ grub_fs_blocklist_open (grub_file_t file, const char *name)
 	}
 
       p++;
-      blocks[i].length = grub_strtoul (p, &p, 0);
+      if (*p == '\0' || *p == ',')
+        blocks[i].length = max_sectors - blocks[i].offset;
+      else
+        blocks[i].length = grub_strtoul (p, &p, 0);
+
       if (grub_errno != GRUB_ERR_NONE
 	  || blocks[i].length == 0
 	  || (*p && *p != ',' && ! grub_isspace (*p)))
-- 
2.27.0

Re: [PATCH] fs: Allow number of blocks in block list to be optional, defaulting length to device length

[Daniel Kiper]

On Sun, Mar 21, 2021 at 01:09:10PM -0500, Glenn Washburn wrote:
> This is primarily useful to do something like "loopback newdev (dev)8+" to
> create a device that skips the first 4K, which may contain a container
> header, eg. a non-standard RAID1 header, that grub does not recognize. This
> would allow that container data to be potentially accessed up to the end of
> container, which may be necessary for some layouts that store data at the
> end. There is currently not a good way to programmatically get the number
> of sectors on a disk to set the appropriate length of the blocklist.
>
> Signed-off-by: Glenn Washburn <development@efficientek.com>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Daniel