[PATCH][next] mm/pgtable: Fix multiple -Wstringop-overflow warnings

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

The actual size of the following arrays at run-time depends on
CONFIG_X86_PAE.

427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];

If CONFIG_X86_PAE is not enabled, their final size will be zero. In that
case, the compiler complains about trying to access objects of size zero
when calling functions where these objects are passed as arguments.

Fix this by sanity-checking the size of those arrays just before the
function calls. Also, the following warnings are fixed by these changes
when building with GCC-11 and -Wstringop-overflow enabled:

arch/x86/mm/pgtable.c:437:13: warning: ‘preallocate_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:440:13: warning: ‘preallocate_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:462:9: warning: ‘free_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:455:9: warning: ‘pgd_prepopulate_user_pmd’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:464:9: warning: ‘free_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/203
Link: https://github.com/KSPP/linux/issues/181
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 arch/x86/mm/pgtable.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 8525f2876fb4..5116df6a308c 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -434,10 +434,12 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 
 	mm->pgd = pgd;
 
-	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
+	if (sizeof(pmds) != 0 &&
+			preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
 		goto out_free_pgd;
 
-	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
+	if (sizeof(u_pmds) != 0 &&
+			preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
 		goto out_free_pmds;
 
 	if (paravirt_pgd_alloc(mm) != 0)
@@ -451,17 +453,22 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 	spin_lock(&pgd_lock);
 
 	pgd_ctor(mm, pgd);
-	pgd_prepopulate_pmd(mm, pgd, pmds);
-	pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
+	if (sizeof(pmds) != 0)
+		pgd_prepopulate_pmd(mm, pgd, pmds);
+
+	if (sizeof(u_pmds) != 0)
+		pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
 
 	spin_unlock(&pgd_lock);
 
 	return pgd;
 
 out_free_user_pmds:
-	free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
+	if (sizeof(u_pmds) != 0)
+		free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
 out_free_pmds:
-	free_pmds(mm, pmds, PREALLOCATED_PMDS);
+	if (sizeof(pmds) != 0)
+		free_pmds(mm, pmds, PREALLOCATED_PMDS);
 out_free_pgd:
 	_pgd_free(pgd);
 out:
-- 
2.34.1