[Libguestfs] [libguestfs-common PATCH 2/2] detect_kernels: deal with RHEL's kernel-core / kernel-modules-core split

[Laszlo Ersek]

In "kernel-5.14.0-269.el9", the "kernel-modules-core" subpackage got split
from the "kernel-core" subpackage. Therefore, a single binary RPM
containing *both* the "/boot/vmlinuz-5.14.0-269.el9.x86_64" file *and* the
"/lib/modules/5.14.0-269.el9.x86_64" directory no longer exists. The file
now belongs to "kernel-core", and the directory to "kernel-modules-core".

As a result, when we investigate the file list of "kernel-core" (based on
"kernel-core" providing "/boot/vmlinuz-5.14.0-269.el9.x86_64"), the first
match against "/lib/modules/" is not the actual module root directory
"/lib/modules/5.14.0-269.el9.x86_64", but the nonsensical
"/lib/modules/5.14.0-269.el9.x86_64/.vmlinuz.hmac" regular file. This
latter file is never a directory, therefore we rule out "kernel-core" as a
kernel package.

We also rule out "kernel-modules-core" (even earlier) because it does not
contain "/boot/vmlinuz-5.14.0-269.el9.x86_64".

Now, the code already deals with the case if the prospective kernel
package *does not provide a match* for the "/lib/modules/" prefix: in that
case, we construct the modpath manually, from said prefix, and the version
number found in "/boot/vmlinuz-<version>". This fallback is good, but it's
unreachable if *there is* a candidate, it's just wrong (i.e., not a
directory).

Perform the "is_dir" check on the candidate modpath earlier, so that we
can fall back to the manual modpath construction if the modpath candidate
exists, but is wrong.

With this, the original "is_dir" check becomes superfluous (duplicated)
*except* when the "Not_found" branch is taken. Therefore, hoist the
original "is_dir" check into that branch.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175703
Reported-by: Vera Wu <vwu at redhat.com>
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
Tested-by: Vera Wu <vwu at redhat.com>
---

Notes:
    context:-U5

 mldrivers/linux_kernels.ml | 24 +++++++++++++++-----
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/mldrivers/linux_kernels.ml b/mldrivers/linux_kernels.ml
index 5b8bdee9923d..23ff76a5504c 100644
--- a/mldrivers/linux_kernels.ml
+++ b/mldrivers/linux_kernels.ml
@@ -125,31 +125,43 @@ let detect_kernels (g : G.guestfs) root bootloader apps =
             *)
            let modpath, version =
              let prefix = "/lib/modules/" in
              let prefix_len = String.length prefix in
              try
-               List.find_map (
+               let modpath, version = List.find_map (
                  fun filename ->
                    let filename_len = String.length filename in
                    if filename_len > prefix_len &&
                       String.is_prefix filename prefix then (
                      let version = String.sub filename prefix_len
                                               (filename_len - prefix_len) in
                      Some (filename, version)
                    ) else
                      None
-               ) files
+               ) files in
+               (* Fall back to the version in the vmlinuz file name not only if
+                * a candidate pathname couldn't be found under /lib/modules/,
+                * but also in case the candidate pathname doesn't reference a
+                * directory. See RHBZ#2175703.
+                *
+                * Note that this "is_dir" check is deliberately kept outside of
+                * the "find_map"'s mapper function above: we want the first
+                * candidate *to be* a directory, and not the first candidate
+                * *that is* a directory.
+                *)
+               if not (g#is_dir ~followsymlinks:true modpath) then
+                 raise Not_found;
+               modpath, version
              with Not_found ->
                let version =
                  String.sub vmlinuz 14 (String.length vmlinuz - 14) in
                let modpath = prefix ^ version in
+               (* Check that the modpath exists. *)
+               if not (g#is_dir ~followsymlinks:true modpath) then
+                 raise Not_found;
                modpath, version in
 
-           (* Check that the modpath exists. *)
-           if not (g#is_dir ~followsymlinks:true modpath) then
-             raise Not_found;
-
            (* Find the initramfs which corresponds to the kernel.
             * Since the initramfs is built at runtime, and doesn't have
             * to be covered by the RPM file list, this is basically
             * guesswork.
             *)