[tip:x86/urgent] x86/mm: Avoid VLA in pgd_alloc()

[tip-bot for Arnd Bergmann <tipbot@xxxxxxxxx>]

Commit-ID:  1be3f247c2882a82279cbcf43717581ea943b692
Gitweb:     https://git.kernel.org/tip/1be3f247c2882a82279cbcf43717581ea943b692
Author:     Arnd Bergmann <arnd@xxxxxxxx>
AuthorDate: Fri, 5 Oct 2018 18:13:16 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 5 Oct 2018 18:16:55 +0200

x86/mm: Avoid VLA in pgd_alloc()

Turning on -Wvla found a new VLA usage:

  arch/x86/mm/pgtable.c: In function 'pgd_alloc':
  include/linux/build_bug.h:29:45: error: ISO C90 forbids variable length array 'u_pmds' [-Werror=vla]
  arch/x86/mm/pgtable.c:190:34: note: in expansion of macro 'static_cpu_has'
   #define PREALLOCATED_USER_PMDS  (static_cpu_has(X86_FEATURE_PTI) ? \
                                    ^~~~~~~~~~~~~~
  arch/x86/mm/pgtable.c:431:16: note: in expansion of macro 'PREALLOCATED_USER_PMDS'
    pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
                ^~~~~~~~~~~~~~~~~~~~~~

Use the actual size of the array that is used for X86_FEATURE_PTI
instead of the variable size.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Joerg Roedel <jroedel@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Toshi Kani <toshi.kani@xxxxxxx>
Fixes: 68664695ae57 ("Makefile: Globally enable VLA warning")
Fixes: f59dbe9ca670 ("x86/pgtable/pae: Use separate kernel PMDs for user page-table")
Link: http://lkml.kernel.org/r/20181005161333.765973-1-arnd@xxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/mm/pgtable.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 089e78c4effd..386b43e3e0ac 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -189,6 +189,7 @@ static void pgd_dtor(pgd_t *pgd)
  */
 #define PREALLOCATED_USER_PMDS	 (static_cpu_has(X86_FEATURE_PTI) ? \
 					KERNEL_PGD_PTRS : 0)
+#define MAX_PREALLOCATED_USER_PMDS KERNEL_PGD_PTRS
 
 void pud_populate(struct mm_struct *mm, pud_t *pudp, pmd_t *pmd)
 {
@@ -211,6 +212,7 @@ void pud_populate(struct mm_struct *mm, pud_t *pudp, pmd_t *pmd)
 /* No need to prepopulate any pagetable entries in non-PAE modes. */
 #define PREALLOCATED_PMDS	0
 #define PREALLOCATED_USER_PMDS	 0
+#define MAX_PREALLOCATED_USER_PMDS 0
 #endif	/* CONFIG_X86_PAE */
 
 static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
@@ -428,8 +430,8 @@ static inline void _pgd_free(pgd_t *pgd)
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
 	pgd_t *pgd;
-	pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
-	pmd_t *pmds[PREALLOCATED_PMDS];
+	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
+	pmd_t *pmds[MAX_PREALLOCATED_USER_PMDS];
 
 	pgd = _pgd_alloc();