Clang builds with defconfig started crashing after commit fb43d6cb91ef
("x86/mm: Do not auto-massage page protections")
This was caused by introducing a new global access in __startup_64().
Code in __startup_64() can be relocated during execution, but the compiler
doesn't have to generate PC-relative relocations when accessing globals
from that function. Clang actually does not generate them, which leads
to boot-time crashes. To work around this problem, every global pointer
must be adjusted using fixup_pointer().
Signed-off-by: Alexander Potapenko <[email protected]>
Fixes: fb43d6cb91ef ("x86/mm: Do not auto-massage page protections")
---
v2: better patch description, added a comment to __startup_64()
---
arch/x86/kernel/head64.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 0c408f8c4ed4..9223792f6d0e 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -104,6 +104,13 @@ static bool __head check_la57_support(unsigned long physaddr)
}
#endif
+
+/* Code in __startup_64() can be relocated during execution, but the compiler
+ * doesn't have to generate PC-relative relocations when accessing globals from
+ * that function. Clang actually does not generate them, which leads to
+ * boot-time crashes. To work around this problem, every global pointer must
+ * be adjusted using fixup_pointer().
+ */
unsigned long __head __startup_64(unsigned long physaddr,
struct boot_params *bp)
{
@@ -113,6 +120,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
p4dval_t *p4d;
pudval_t *pud;
pmdval_t *pmd, pmd_entry;
+ pteval_t *mask_ptr;
bool la57;
int i;
unsigned int *next_pgt_ptr;
@@ -196,7 +204,8 @@ unsigned long __head __startup_64(unsigned long physaddr,
pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
/* Filter out unsupported __PAGE_KERNEL_* bits: */
- pmd_entry &= __supported_pte_mask;
+ mask_ptr = (pteval_t *)fixup_pointer(&__supported_pte_mask, physaddr);
+ pmd_entry &= *mask_ptr;
pmd_entry += sme_get_me_mask();
pmd_entry += physaddr;
--
2.17.0.441.gb46fe60e1d-goog
On 05/08/2018 09:28 AM, Alexander Potapenko wrote:
> Clang builds with defconfig started crashing after commit fb43d6cb91ef
> ("x86/mm: Do not auto-massage page protections")
> This was caused by introducing a new global access in __startup_64().
>
> Code in __startup_64() can be relocated during execution, but the compiler
> doesn't have to generate PC-relative relocations when accessing globals
> from that function. Clang actually does not generate them, which leads
> to boot-time crashes. To work around this problem, every global pointer
> must be adjusted using fixup_pointer().
Looks good to me. Thanks for adding the comment, especially!
Reviewed-by: Dave Hansen <[email protected]>
On Tue, May 08, 2018 at 04:28:29PM +0000, Alexander Potapenko wrote:
> @@ -196,7 +204,8 @@ unsigned long __head __startup_64(unsigned long physaddr,
>
> pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
> /* Filter out unsupported __PAGE_KERNEL_* bits: */
> - pmd_entry &= __supported_pte_mask;
> + mask_ptr = (pteval_t *)fixup_pointer(&__supported_pte_mask, physaddr);
Do we really need the cast here?
--
Kirill A. Shutemov
On Tue, May 8, 2018 at 11:44 PM Kirill A. Shutemov <
[email protected]> wrote:
> On Tue, May 08, 2018 at 04:28:29PM +0000, Alexander Potapenko wrote:
> > @@ -196,7 +204,8 @@ unsigned long __head __startup_64(unsigned long
physaddr,
> >
> > pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
> > /* Filter out unsupported __PAGE_KERNEL_* bits: */
> > - pmd_entry &= __supported_pte_mask;
> > + mask_ptr = (pteval_t *)fixup_pointer(&__supported_pte_mask,
physaddr);
> Do we really need the cast here?
Correct, we do not.
> --
> Kirill A. Shutemov
--
Alexander Potapenko
Software Engineer
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