On Thu, Aug 21, 2025 at 11:10:39AM +0100, Lorenzo Stoakes wrote:
On Thu, Aug 21, 2025 at 06:35:42PM +0900, Harry Yoo wrote:
KASAN unconditionally references kasan_early_shadow_{p4d,pud}. However, these global variables may not exist depending on the number of page table levels. For example, if CONFIG_PGTABLE_LEVELS=3, both variables do not exist. Although KASAN may refernce non-existent variables, it didn't break builds because calls to {pgd,p4d}_populate() are optimized away at compile time.
However, {pgd,p4d}_populate_kernel() is defined as a function regardless of the number of page table levels, so the compiler may not optimize them away. In this case, the following linker error occurs:
ld.lld: error: undefined symbol: kasan_early_shadow_p4d
referenced by init.c:260 (/home/hyeyoo/mm-new/mm/kasan/init.c:260) mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a referenced by init.c:260 (/home/hyeyoo/mm-new/mm/kasan/init.c:260) mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a did you mean: kasan_early_shadow_pmd defined in: vmlinux.a(mm/kasan/init.o)
ld.lld: error: undefined symbol: kasan_early_shadow_pud
referenced by init.c:263 (/home/hyeyoo/mm-new/mm/kasan/init.c:263) mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a referenced by init.c:263 (/home/hyeyoo/mm-new/mm/kasan/init.c:263) mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a referenced by init.c:200 (/home/hyeyoo/mm-new/mm/kasan/init.c:200) mm/kasan/init.o:(zero_p4d_populate) in archive vmlinux.a referenced 1 more times
Therefore, to allow calls to {pgd,p4d}_populate_kernel() to be optimized out at compile time, define {pgd,p4d}_populate_kernel() as macros. This way, when pgd_populate() or p4d_populate() are simply empty macros, the corresponding *_populate_kernel() functions can also be optimized away.
Signed-off-by: Harry Yoo harry.yoo@oracle.com
This looks good, other than the nit below re: a comment, I think when we are doing this kind of thing it's necessary to spell out plainly why exactly we're doing it because it's not obvious at first glance.
Good point, will do:
/* * {pgd,p4d}_populate_kernel() are defined as macros to allow * compile-time optimization based on the configured page table levels. * Without this, linking may fail because callers (e.g., KASAN) may rely * on calls to these functions being optimized away when passing symbols * that exist only for certain page table levels. */
Anyway have checked locally and all good and LGTM code-wise so aside from above:
Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com
Thanks!