[PATCH] ARM: fix string functions on !MMU

[rabin@rab.in (Rabin Vincent)]

On Fri, Apr 25, 2014 at 10:12:24AM +0100, Will Deacon wrote:
> On Thu, Apr 24, 2014 at 04:43:20PM +0100, Rabin Vincent wrote:
> > On Tue, Apr 22, 2014 at 10:44:24AM +0100, Will Deacon wrote:
> > > On Mon, Apr 21, 2014 at 07:10:08PM +0100, Rabin Vincent wrote:
> > > > 8c56cc8be5b38e ("ARM: 7449/1: use generic strnlen_user and
> > > > strncpy_from_user functions") apparently broken those string operations
> > > > for !MMU.  USER_DS == KERNEL_DS on !MMU, so user_addr_max() always
> > > > restricts the addresses to TASK_SIZE.
> > > > 
> > > > TASK_SIZE has anyway no meaning on !MMU, so make user_addr_max() not
> > > > restrict anything.
> > > 
> > > Might be worth mentioning that this is an issue because KERNEL_DS is 0x0
> > > (since it's a 32-bit quantity), so checks like addr < user_addr_max() will
> > > fail.
> > 
> > Thanks for the ack, but I don't quite understand what you mean here.
> > You describe the state before this patch, right?  Why does it matter
> > that KERNEL_DS is 0x0?
> 
> Apologies, I misread the code that you're patching so I guess I'll have to
> revoke my ack, sorry! That's not to say I think the patch is bad, I'd just
> like to discuss it with you a bit more.
> 
> Having re-read the code, the issue is because TASK_SIZE is defined as
> CONFIG_DRAM_SIZE, right? Since CONFIG_DRAM_BASE may be non-zero, it means
> TASK_SIZE is truly a size -- not a limit (as it would be in virtual space).
> What we actually want for !MMU is END_MEM instead of TASK_SIZE.

I guess you mean I should #define user_addr_max() to END_MEM for !MMU?
That won't work.  For example, on a !MMU boot, the first thing that
fails because of this bug is devtmpsfs_mount() -> sys_mount() ->
copy_mount_string() -> strndup_user(), which is run in a kernel thread.
The type argument of sys_mount() points to a string in flash since we
run an XIP kernel.  This flash address has no relation to END_MEM (which
is nothing but CONFIG_DRAM_BASE + CONFIG_DRAM_SIZE) and could be higher
than END_MEM.