[PATCH 0/1] PPC32: fix ptrace() access to FPU registers

[Christophe Leroy]

Le 19/06/2019 à 14:57, Radu Rendec a écrit :
> On Wed, 2019-06-19 at 10:36 +1000, Daniel Axtens wrote:
>> Andreas Schwab <
>> schwab at linux-m68k.org
>>> writes:
>>
>>> On Jun 18 2019, Radu Rendec <
>>> radu.rendec at gmail.com
>>>> wrote:
>>>
>>>> Since you already have a working setup, it would be nice if you could
>>>> add a printk to arch_ptrace() to print the address and confirm what I
>>>> believe happens (by reading the gdb source code).
>>>
>>> A ppc32 ptrace syscall goes through compat_arch_ptrace.
> 
> Right. I completely overlooked that part.
> 
>> Ah right, and that (in ptrace32.c) contains code that will work:
>>
>>
>> 			/*
>> 			 * the user space code considers the floating point
>> 			 * to be an array of unsigned int (32 bits) - the
>> 			 * index passed in is based on this assumption.
>> 			 */
>> 			tmp = ((unsigned int *)child->thread.fp_state.fpr)
>> 				[FPRINDEX(index)];
>>
>> FPRINDEX is defined above to deal with the various manipulations you
>> need to do.
> 
> Correct. Basically it does the same that I did in my patch: it divides
> the index again by 2 (it's already divided by 4 in compat_arch_ptrace()
> so it ends up divided by 8), then takes the least significant bit and
> adds it to the index. I take bit 2 of the original address, which is the
> same thing (because in FPRHALF() the address is already divided by 4).
> 
> So we have this in ptrace32.c:
> 
> #define FPRNUMBER(i) (((i) - PT_FPR0) >> 1)
> #define FPRHALF(i) (((i) - PT_FPR0) & 1)
> #define FPRINDEX(i) TS_FPRWIDTH * FPRNUMBER(i) * 2 + FPRHALF(i)
> 
> index = (unsigned long) addr >> 2;
> (unsigned int *)child->thread.fp_state.fpr)[FPRINDEX(index)]
> 
> 
> And we have this in my patch:
> 
> fpidx = (addr - PT_FPR0 * sizeof(long)) / 8;
> (void *)&child->thread.TS_FPR(fpidx) + (addr & 4)
> 
>> Radu: I think we want to copy that working code back into ptrace.c.
> 
> I'm not sure that would work. There's a subtle difference: the code in
> ptrace32.c is always compiled on a 64-bit kernel and the user space
> calling it is always 32-bit; on the other hand, the code in ptrace.c can
> be compiled on either a 64-bit kernel or a 32-bit kernel and the user
> space calling it always has the same "bitness" as the kernel.
> 
> One difference is the size of the CPU registers. On 64-bit they are 8
> byte long and user space knows that and generates 8-byte aligned
> addresses. So you have to divide the address by 8 to calculate the CPU
> register index correctly, which compat_arch_ptrace() currently doesn't.
> 
> Another difference is that on 64-bit `long` is 8 bytes, so user space
> can read a whole FPU register in a single ptrace call.
> 
> Now that we are all aware of compat_arch_ptrace() (which handles the
> special case of a 32-bit process running on a 64-bit kernel) I would say
> the patch is correct and does the right thing for both 32-bit and 64-bit
> kernels and processes.
> 
>> The challenge will be unpicking the awful mess of ifdefs in ptrace.c
>> and making it somewhat more comprehensible.
> 
> I'm not sure what ifdefs you're thinking about. The only that are used
> inside arch_ptrace() are PT_FPR0, PT_FPSCR and TS_FPR, which seem to be
> correct.
> 
> But perhaps it would be useful to change my patch and add a comment just
> before arch_ptrace() that explains how the math is done and that the
> code must work on both 32-bit and 64-bit, the user space address
> assumptions, etc.
> 
> By the way, I'm not sure the code in compat_arch_ptrace() handles
> PT_FPSCR correctly. It might (just because fpscr is right next to fpr[]
> in memory - and that's a hack), but I can't figure out if it accesses
> the right half.
> 


Does the issue still exists ? If yes, the patch has to be rebased.