ARM: gcc5 causes undefined behaviour in mmu_init()

[Enrico Scholz <enrico.scholz@sigma-chemnitz.de>]

Hi,

a barebox built with gcc5 hangs in tlb_invalidate() very early.  This
seems to be caused by an integer overflow:

--- arch/arm/cpu/mmu.c:
static int arm_mmu_remap_sdram(struct memory_bank *bank)
{
	unsigned long num_ptes = bank->size >> 10;
	int i;

	for (i = 0; i < num_ptes; i++) {
		ptes[i] = (phys + i * PAGE_SIZE) | PTE_TYPE_SMALL |
			pte_flags_cached;
	}


For 1GiB RAM, 'num_ptes' is 1MiB and due to integer promotion, the 'i *
PAGE_SIZE' overflows and causes undefined behavior.

A trivial fix is to make 'i' an unsigned int or long.


But I wonder whether calculation of 'num_ptes' is really correct.  Does
barebox really use a 1KiB pagesize for PTEs or should the '>> 10' be a
'>> 12'?  When it is really 1KiB, the mapping for memory sizes > 1GiB
seem to be ambiguous then.


For reference; in broken case ('int i'), gcc5 generates:

|   4badc:       f7b4 fe7e       bl      7dc <pr_print>
|   4bae0:       f8d9 3000       ldr.w   r3, [r9]
|   4bae4:       f1aa 0104       sub.w   r1, sl, #4
|   4bae8:       f043 0202       orr.w   r2, r3, #2
|   4baec:       9b03            ldr     r3, [sp, #12]
|   4baee:       eb04 3303       add.w   r3, r4, r3, lsl #12
|
|   4baf2:       42a3            cmp     r3, r4
|   4baf4:       d006            beq.n   4bb04 <mmu_init+0x1c0>
|   4baf6:       ea42 0004       orr.w   r0, r2, r4
|   4bafa:       f504 5480       add.w   r4, r4, #4096   ; 0x1000
|   4bafe:       f841 0f04       str.w   r0, [r1, #4]!
|   4bb02:       e7f6            b.n     4baf2 <mmu_init+0x1ae>

Building with 'unsigned long i' generates:

|   4bad8:       f7b4 fe80       bl      7dc <pr_print>
|   4badc:       683b            ldr     r3, [r7, #0]
|   4bade:       f043 0202       orr.w   r2, r3, #2
|   4bae2:       9b05            ldr     r3, [sp, #20]
|
|   4bae4:       9904            ldr     r1, [sp, #16]
|   4bae6:       4299            cmp     r1, r3
|   4bae8:       d006            beq.n   4baf8 <mmu_init+0x1b4>
|   4baea:       eb0a 3103       add.w   r1, sl, r3, lsl #12
|   4baee:       4311            orrs    r1, r2
|   4baf0:       f849 1023       str.w   r1, [r9, r3, lsl #2]
|   4baf4:       3301            adds    r3, #1
|   4baf6:       e7f5            b.n     4bae4 <mmu_init+0x1a0>


Enrico
-- 
SIGMA Chemnitz GmbH       Registergericht:   Amtsgericht Chemnitz HRB 1750
Am Erlenwald 13           Geschaeftsfuehrer: Grit Freitag, Frank Pyritz
09128 Chemnitz


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