[PATCH] gzio: Avoid spurious failures

[Jason Andryuk]

grub_gzio_read_real() uses grub_errno to identify decompression failures
when the function completes.  Its callees in gzio.c are void-returning
functions that set grub_errno on their exit paths.

A Fedora 38 machine was observed to fail `multiboot2 /xen.gz` with
"premature end of file".  xen.gz was well formed and could be
successfully gunzip-ed in Linux.

The TPM is flaky and produces errors when the verifier tries to extend
PCRs with measurements.  Logs show "Unkown TPM error" and grub_errno is
set to GRUB_ERR_UNKNOWN_DEVICE.  This pre-existing grub_errno causes an
otherwise successful grub_gzio_read_real() call to return error.

Clear grub_errno at the start of the function to avoid such errors.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
---
 grub-core/io/gzio.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/grub-core/io/gzio.c b/grub-core/io/gzio.c
index ca9355751..1e31a8255 100644
--- a/grub-core/io/gzio.c
+++ b/grub-core/io/gzio.c
@@ -1294,6 +1294,14 @@ grub_gzio_read_real (grub_gzio_t gzio, grub_off_t offset,
 {
   grub_ssize_t ret = 0;
 
+  /* Avoid spurious failures on exit when grub_errno is already set. */
+  if (grub_errno != GRUB_ERR_NONE)
+    {
+      grub_dprintf ("gzio", "%s: clearing pre-exising errmsg %s\n",
+                    __func__, grub_errmsg);
+      grub_errno = GRUB_ERR_NONE;
+    }
+
   /* Do we reset decompression to the beginning of the file?  */
   if (gzio->saved_offset > offset + WSIZE)
     initialize_tables (gzio);
-- 
2.41.0