TASK_SIZE for !MMU

[Greg Ungerer]

Hi Uwe,

On 02/06/14 18:51, Uwe Kleine-König wrote:
> Hello
>
> [expand Cc: a bit]
>
> On Tue, Apr 29, 2014 at 12:00:28PM +0200, Uwe Kleine-König wrote:
>> I grepped through the kernel (v3.15-rc1) for usages of TASK_SIZE to
>> check if/how it is used on !MMU ARM machines. Most open questions also
>> affect the other !MMU platforms, so I put the blackfin, c6x, frv and
>> m32r, m68k, microblaze and xtensa lists on Cc:. (Did I miss a platform
>> that cares for !MMU ?)
>>
>> Most occurences are fine, see the list at the end of this mail. However
>> some are not or are unclear to me. Here is the complete list[1] apart from
>> the definition of TASK_SIZE for !MMU in arch/arm/include/asm/memory.h:
>>
>>   - Probably this should be explict s/TASK_SIZE/CONFIG_DRAM_SIZE/. This
>>     is generic code however while CONFIG_DRAM_SIZE is ARM only.
>>          mm/nommu.c:     if (!rlen || rlen > TASK_SIZE)
>>
>>   - The issue the patch by Rabin is addressing (Subject: [PATCH] ARM: fix
>>     string functions on !MMU), alternatively make TASK_SIZE ~0UL.
>>          arch/arm/include/asm/uaccess.h:#define user_addr_max() \
>>          arch/arm/include/asm/uaccess.h: (segment_eq(get_fs(), USER_DS) ? TASK_SIZE : ~0UL)
> [reference: http://www.spinics.net/lists/arm-kernel/msg324112.html ]
>
>>   - probably bearable if broken:
>>          drivers/misc/lkdtm.c:           if (user_addr >= TASK_SIZE) {
>>          lib/test_user_copy.c:   user_addr = vm_mmap(...)
>>          lib/test_user_copy.c:   if (user_addr >= (unsigned long)(TASK_SIZE)) {
>>          lib/test_user_copy.c:           pr_warn("Failed to allocate user memory\n");
>>          lib/test_user_copy.c:           return -ENOMEM;
>>
>>   - unclear to me:
>>          fs/exec.c:      current->mm->task_size = TASK_SIZE;
>>     - depends on PERF_EVENTS
>>          kernel/events/core.c:   if (!addr || addr >= TASK_SIZE)
>>          kernel/events/core.c:   return TASK_SIZE - addr;
>>          kernel/events/uprobes.c:                area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
>>     - depends on (PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)):
>>          arch/arm/kernel/hw_breakpoint.c:        return (va >= TASK_SIZE) && ((va + len - 1) >= TASK_SIZE);
>>     - seems to cope with big TASK_SIZE
>>          fs/namespace.c:        size = TASK_SIZE - (unsigned long)data;
>>          fs/namespace.c:        if (size > PAGE_SIZE)
>>          fs/namespace.c:                size = PAGE_SIZE;
>>     - depends on PLAT_S5P || ARCH_EXYNOS, this looks wrong
>>          drivers/media/platform/s5p-mfc/s5p_mfc_common.h:#define DST_QUEUE_OFF_BASE      (TASK_SIZE / 2)
>>     - used for prctl(PR_SET_MM, ...)
>>          kernel/sys.c:   if (addr >= TASK_SIZE || addr < mmap_min_addr)
>>
>> Any help to judge if these are OK is appreciated (even from Will :-)
>>
>> I think it would be OK to define TASK_SIZE to 0xffffffff for !MMU.
>> blackfin, frv and m68k also do this. c6x does define it to 0xFFFFF000 to
>> leave space for error codes.

I did that same change for m68k in commit cc24c40 ("m68knommu: remove
size limit on non-MMU TASK_SIZE"). For similar reasons as you need to
now.


>> Thoughts?
> The problem is that current linus/master (and also next) doesn't boot on
> my ARM-nommu machine because the user string functions (strnlen_user,
> strncpy_from_user et al.) refuse to work on strings above TASK_SIZE
> which in my case also includes the XIP kernel image.

I seem to recall that we were not considering flash or anything else
other than RAM when defining that original TASK_SIZE (back many, many
years ago). Some of the address checks you list above made some sense
if you had everything in RAM (though only upper bounds are checked).
The thinking was some checking is better than none I suppose.

Setting a hard coded memory size in CONFIG_DRAM_SIZE is not all that
fantastic either...

Regards
Greg


> Maybe someone of the mm people can bring light into the unclear points
> above and the question what TASK_SIZE is supposed to be on no-MMU
> machines?
>
> Best regards
> Uwe
>
>> [1] complete as in "skip everything below arch/ but arch/arm" :-)
>>
> [removed the list, if you're interested, it's available at
> http://mid.gmane.org/20140429100028.GH28564@pengutronix.de]
>