[PATCH v2] x86/boot/e820: Re-enable fallback if e820 table is empty

[David Gow <david@davidgow.net>]

In commit 157266edcc56 ("x86/boot/e820: Simplify append_e820_table() and
remove restriction on single-entry tables"), the check that the number of
entries in the e820 table was removed. The intention was to support
single-entry maps, but by removing the check entirely, we also skip the
fallback (to, e.g., the BIOS 88h function).

This means that if no E820 map is passed in from the bootloader (which is
the case on some bootloaders, like linld), we end up with an empty memory
map, and the kernel fails to boot (either by deadlocking on OOM, or by
failing to allocate the real mode trampoline, or similar).

Re-instate the check in append_e820_table(), but only check nr_entries is
non-zero. This allows e820__memory_setup_default() to fall back to other
memory size sources, and doesn't affect e820__memory_setup_extended(), as
the latter ignores the return value from append_e820_table().

In so doing, we also update the return values to be proper error codes,
with -ENOENT for this case (there are no entries), and -EINVAL for the case
where an entry appears invalid. Given none of the callers check the actual
value -- just whether it's nonzero -- this is largely aesthetic in
practice.

Tested against linld, and the kernel boots again fine.

Fixes: 157266edcc56 ("x86/boot/e820: Simplify append_e820_table() and remove restriction on single-entry tables")
Signed-off-by: David Gow <david@davidgow.net>
---

Changes since v2:
https://lore.kernel.org/lkml/20260415003021.1543723-1-david@davidgow.net/
- Return -ENOENT instead of -1
- Return -EINVAL instead of -1 for the case where an entry looks invalid due to
  an overflow (Thanks, Andy)

---
 arch/x86/kernel/e820.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 2a9992758933..3db9c364b833 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -450,6 +450,10 @@ __init static int append_e820_table(struct boot_e820_entry *entries, u32 nr_entr
 {
 	struct boot_e820_entry *entry = entries;
 
+	/* If there aren't any entries, we'll want to fall-back to another source. */
+	if (!nr_entries)
+		return -ENOENT;
+
 	while (nr_entries) {
 		u64 start = entry->addr;
 		u64 size  = entry->size;
@@ -458,7 +462,7 @@ __init static int append_e820_table(struct boot_e820_entry *entries, u32 nr_entr
 
 		/* Ignore the remaining entries on 64-bit overflow: */
 		if (start > end && likely(size))
-			return -1;
+			return -EINVAL;
 
 		e820__range_add(start, size, type);
 
-- 
2.53.0