Subject: Re: linux-next: fix ups for clashes between akpm and powerpc trees

From: Michael Ellerman <[email-masked]>
Date: Thu, 04 Jun 2020 21:28:23 +1000

Stephen Rothwell <[email-masked]> writes:
> Hi all,
>
> On Thu, 4 Jun 2020 16:52:46 +1000 Stephen Rothwell <[email-masked]> wrote:
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 25c3cb8272c0..a6799723cd98 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -1008,6 +1008,12 @@ extern struct page *p4d_page(p4d_t p4d);
>>  #define pud_page_vaddr(pud)	__va(pud_val(pud) & ~PUD_MASKED_BITS)
>>  #define p4d_page_vaddr(p4d)	__va(p4d_val(p4d) & ~P4D_MASKED_BITS)
>>  
>> +static inline unsigned long pgd_index(unsigned long address)
>> +{
>> +	return (address >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1);
>> +}
>> +#define pgd_index pgd_index
>> +
>>  #define pte_ERROR(e) \
>>  	pr_err("%s:%d: bad pte %08lx.\n", __FILE__, __LINE__, pte_val(e))
>>  #define pmd_ERROR(e) \
>
> I have added that hunk to linux-next for tomorrow as a fix for
> mm-consolidate-pgd_index-and-pgd_offset_k-definitions.
>
> Its not strickly necessary, but Michael expressed a preference for the
> inline function.

That was because we just recently converted it into a static inline to
avoid UBSAN warnings:

commit c2e929b18cea6cbf71364f22d742d9aad7f4677a
Author:     Qian Cai <[email-masked]>
AuthorDate: Thu Mar 5 23:48:52 2020 -0500

    powerpc/64s/pgtable: fix an undefined behaviour

    Booting a power9 server with hash MMU could trigger an undefined
    behaviour because pud_offset(p4d, 0) will do,

    0 >> (PAGE_SHIFT:16 + PTE_INDEX_SIZE:8 + H_PMD_INDEX_SIZE:10)

    Fix it by converting pud_index() and friends to static inline
    functions.

    UBSAN: shift-out-of-bounds in arch/powerpc/mm/ptdump/ptdump.c:282:15
    shift exponent 34 is too large for 32-bit type 'int'
    CPU: 6 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-next-20200303+ #13
    Call Trace:
    dump_stack+0xf4/0x164 (unreliable)
    ubsan_epilogue+0x18/0x78
    __ubsan_handle_shift_out_of_bounds+0x160/0x21c
    walk_pagetables+0x2cc/0x700
    walk_pud at arch/powerpc/mm/ptdump/ptdump.c:282
    (inlined by) walk_pagetables at arch/powerpc/mm/ptdump/ptdump.c:311
    ptdump_check_wx+0x8c/0xf0
    mark_rodata_ro+0x48/0x80
    kernel_init+0x74/0x194
    ret_from_kernel_thread+0x5c/0x74

> I was wondering if pgd_index "Must be a compile-time
> constant" on one (or a few) architectures, then why not leave the
> default as an inline function and special case it as a macro where
> needed ...

AIUI that requirement comes from x86 which has:

#define KERNEL_PGD_BOUNDARY	pgd_index(PAGE_OFFSET)
#define KERNEL_PGD_PTRS		(PTRS_PER_PGD - KERNEL_PGD_BOUNDARY)
...
#define MAX_PREALLOCATED_USER_PMDS KERNEL_PGD_PTRS
...
pgd_t *pgd_alloc(struct mm_struct *mm)
{
	pgd_t *pgd;
	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];

Which will produce a variable length array if pgd_index() isn't a
compile-time constant.

cheers