[PATCH v2] module: Make sure relocations are applied to the per-CPU section

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

The per-CPU data section is handled differently than the other sections.
The memory allocations requires a special __percpu pointer and then the
section is copied into the view of each CPU. Therefore the SHF_ALLOC
flag is removed to ensure move_module() skips it.

Later, relocations are applied and apply_relocations() skips sections
without SHF_ALLOC because they have not been copied. This also skips the
per-CPU data section.
The missing relocations result in a NULL pointer on x86-64 and very
small values on x86-32. This results in a crash because it is not
skipped like NULL pointer would and can't be dereferenced.

Such an assignment happens during static per-CPU lock initialisation
with lockdep enabled.

Add the SHF_ALLOC flag back for the per-CPU section (if found) after
move_module().

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202506041623.e45e4f7d-lkp@intel.com
Fixes: 8d8022e8aba85 ("module: do percpu allocation after uniqueness check.  No, really!")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v1…v2: https://lore.kernel.org/all/20250604152707.CieD9tN0@linutronix.de/
  - Add the flag back only on SMP if the per-CPU section was found.

 kernel/module/main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 5c6ab20240a6d..4f6554dedf8ea 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2816,6 +2816,10 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	if (err)
 		return ERR_PTR(err);
 
+	/* Add SHF_ALLOC back so that relocations are applied. */
+	if (IS_ENABLED(CONFIG_SMP) && info->index.pcpu)
+		info->sechdrs[info->index.pcpu].sh_flags |= SHF_ALLOC;
+
 	/* Module has been copied to its final place now: return it. */
 	mod = (void *)info->sechdrs[info->index.mod].sh_addr;
 	kmemleak_load_module(mod, info);
-- 
2.49.0