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 <
sch...@linux-m68k.org
writes:
On Jun 18 2019, Radu Rendec <
radu.ren...@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.
