[PATCH v3] 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.

Allow relocation processing for the per-CPU section even if SHF_ALLOC is
missing.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202506041623.e45e4f7d-lkp@intel.com
Fixes: 1a6100caae425 ("Don't relocate non-allocated regions in modules.") #v2.6.1-rc3
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v2…v3: https://lore.kernel.org/all/20250605060738.SzA3UESe@linutronix.de/
  - Check for assigned per-CPU section instead of adding the SHF_ALLOC
    flag back.

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 | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 413ac6ea37021..cb281e82226cd 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -1573,8 +1573,14 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
 		if (infosec >= info->hdr->e_shnum)
 			continue;
 
-		/* Don't bother with non-allocated sections */
-		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
+		/*
+		 * Don't bother with non-allocated sections.
+		 * An exception is the percpu section, which has separate allocations
+		 * for individual CPUs. We relocate the percpu section in the initial
+		 * ELF template and subsequently copy it to the per-CPU destinations.
+		 */
+		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC) &&
+		    (!infosec || infosec != info->index.pcpu))
 			continue;
 
 		if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
-- 
2.49.0