[PATCH] kern/main: Fix cmdpath in root directory

[Michael Chang]

The "cmdpath" environment variable is set at startup to the location
from which the grub image is loaded. It includes a device part and,
optionally, an absolute directory name if the grub image is booted as a
file in a local file-system directory, or in a remote server directory,
like TFTP.

This entire process relies on firmware to provide the correct device
path of the booted image.

We encountered an issue when the image is booted from the root
directory, where the absolute directory name "/" is discarded. This
makes it unclear whether the root path was missing in the firmware
provided device path or if it is simply the root directory. This
ambiguity can cause confusion in custom scripts, potentially causing
them to interpret firmware data incorrectly and trigger unintended
fallback measures.

This patch fixes the problem by properly assigning the "fwpath" returned
by "grub_machine_get_bootlocation()" to "cmdpath". The fix is based on
the fact that fwpath is NULL if the firmware didn’t provide a path part
or an NUL character, "", if it represents the root directory. With this,
it becomes possible to clearly distinguish:

- cmdpath=(hd0,1) - Either the image is booted from the first (raw)
  partition, or the firmware failed to provide the path part.
- cmdpath=(hd0,1)/ - The image is booted from the root directory in the
  first partition.

As a side note, the fix is similar to [1], but without the renaming
part.

[1] https://mail.gnu.org/archive/html/grub-devel/2024-10/msg00155.html

Signed-off-by: Michael Chang <mchang@suse.com>
---
 grub-core/kern/main.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/grub-core/kern/main.c b/grub-core/kern/main.c
index d29494d54..6645173c1 100644
--- a/grub-core/kern/main.c
+++ b/grub-core/kern/main.c
@@ -136,7 +136,11 @@ grub_set_prefix_and_root (void)
     {
       char *cmdpath;
 
-      cmdpath = grub_xasprintf ("(%s)%s", fwdevice, fwpath ? : "");
+      if (fwpath && *fwpath == '\0')
+       cmdpath = grub_xasprintf ("(%s)/", fwdevice);
+      else
+       cmdpath = grub_xasprintf ("(%s)%s", fwdevice, fwpath ? : "");
+
       if (cmdpath)
        {
          grub_env_set ("cmdpath", cmdpath);
-- 
2.47.0