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

[Daniel Axtens]

Radu Rendec <radu.rendec at gmail.com> writes:

> Hi Everyone,
>
> I'm following up on the ptrace() problem that I reported a few days ago.
> I believe my version of the code handles all cases correctly. While the
> problem essentially boils down to dividing the fpidx by 2 on PPC32, it
> becomes tricky when the same code must work correctly on both PPC32 and
> PPC64.
>
> One other thing that I believe was handled incorrectly in the previous
> version is the unused half of fpscr on PPC32. Note that while PT_FPSCR
> is defined as (PT_FPR0 + 2*32 + 1), making only the upper half visible,
> PT_FPR0 + 2*32 still corresponds to a possible address that userspace
> can pass. In that case, comparing fpidx to (PT_FPSCR - PT_FPR0) would
> cause an invalid access to the FPU registers array.
>
> I tested the patch on 4.9.179, but that part of the code is identical in
> recent kernels so it should work just the same.
>
> I wrote a simple test program than can be used to quickly test (on an
> x86_64 host) that all cases are handled correctly for both PPC32/PPC64.
> The code is included below.
>
> I also tested with gdbserver (test patch included below) and verified
> that it generates two ptrace() calls for each FPU register, with
> addresses between 0xc0 and 0x1bc.

Thanks for looking in to this. I can confirm your issue. What I'm
currently wondering is: what is the behaviour with a 32-bit userspace on
a 64-bit kernel? Should they also be going down the 32-bit path as far
as calculating offsets goes?

Regards,
Daniel

>
> 8<--------------- Makefile ---------------------------------------------
> .PHONY: all clean
>
> all: ptrace-fpregs-32 ptrace-fpregs-64
>
> ptrace-fpregs-32: ptrace-fpregs.c
> 	$(CC) -o ptrace-fpregs-32 -Wall -O2 -m32 ptrace-fpregs.c
>
> ptrace-fpregs-64: ptrace-fpregs.c
> 	$(CC) -o ptrace-fpregs-64 -Wall -O2 ptrace-fpregs.c
>
> clean:
> 	rm -f ptrace-fpregs-32 ptrace-fpregs-64
> 8<--------------- ptrace-fpregs.c --------------------------------------
> #include <stdio.h>
> #include <errno.h>
>
> #define PT_FPR0	48
>
> #ifndef __x86_64
>
> #define PT_FPR31 (PT_FPR0 + 2*31)
> #define PT_FPSCR (PT_FPR0 + 2*32 + 1)
>
> #else
>
> #define PT_FPSCR (PT_FPR0 + 32)
>
> #endif
>
> int test_access(unsigned long addr)
> {
> 	int ret;
>
> 	do {
> 		unsigned long index, fpidx;
>
> 		ret = -EIO;
>
> 		/* convert to index and check */
> 		index = addr / sizeof(long);
> 		if ((addr & (sizeof(long) - 1)) || (index > PT_FPSCR))
> 			break;
>
> 		if (index < PT_FPR0) {
> 			ret = printf("ptrace_put_reg(%lu)", index);
> 			break;
> 		}
>
> 		ret = 0;
> #ifndef __x86_64
> 		if (index == PT_FPSCR - 1) {
> 			/* corner case for PPC32; do nothing */
> 			printf("corner_case");
> 			break;
> 		}
> #endif
> 		if (index == PT_FPSCR) {
> 			printf("fpscr");
> 			break;
> 		}
>
> 		/*
> 		 * FPR is always 64-bit; on PPC32, userspace does two 32-bit
> 		 * accesses. Add bit2 to allow accessing the upper half on
> 		 * 32-bit; on 64-bit, bit2 is always 0 (we validate it above).
> 		 */
> 		fpidx = (addr - PT_FPR0 * sizeof(long)) / 8;
> 		printf("TS_FPR[%lu] + %lu", fpidx, addr & 4);
> 		break;
> 	} while (0);
>
> 	return ret;
> }
>
> int main(void)
> {
> 	unsigned long addr;
> 	int rc;
>
> 	for (addr = 0; addr < PT_FPSCR * sizeof(long) + 16; addr++) {
> 		printf("0x%04lx: ", addr);
> 		rc = test_access(addr);
> 		if (rc < 0)
> 			printf("!err!");
> 		printf("\t<%d>\n", rc);
> 	}
>
> 	return 0;
> }
> 8<--------------- gdb.patch --------------------------------------------
> --- gdb/gdbserver/linux-low.c.orig	2019-06-10 11:45:53.810882669 -0400
> +++ gdb/gdbserver/linux-low.c	2019-06-10 11:49:32.272929766 -0400
> @@ -4262,6 +4262,8 @@ store_register (struct regcache *regcach
>    pid = lwpid_of (get_thread_lwp (current_inferior));
>    for (i = 0; i < size; i += sizeof (PTRACE_XFER_TYPE))
>      {
> +      printf("writing register #%d offset %d at address %#x\n",
> +             regno, i, (unsigned int)regaddr);
>        errno = 0;
>        ptrace (PTRACE_POKEUSER, pid,
>  	    /* Coerce to a uintptr_t first to avoid potential gcc warning
> 8<----------------------------------------------------------------------
>
> Radu Rendec (1):
>   PPC32: fix ptrace() access to FPU registers
>
>  arch/powerpc/kernel/ptrace.c | 85 ++++++++++++++++++++++--------------
>  1 file changed, 52 insertions(+), 33 deletions(-)
>
> -- 
> 2.20.1