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SubjectRE: ptrace bug
Date
I don't think the problem is what you identified.
arch/i386/kernel/ptrace.c:getreg() takes the extra FS,GS offsets into
account and performs special handling for them. I suspect this is because
they are not stable per-thread registers and not saved into the register
structure on an interrupt or system call.

ptrace.c:getreg() ...
switch (regno >> 2) {
case FS:
retval = child->thread.fs;
break;
case GS:
retval = child->thread.gs;
break;
case DS:
case ES:
case SS:
case CS:
retval = 0xffff;
/* fall through */
default:
if (regno > GS*4) // *** Adusts for missing FS,GS fields *** //
regno -= 2*4;
regno = regno - sizeof(struct pt_regs);
retval &= get_stack_long(child, regno);
}

-----Original Message-----
From: linux-kernel-owner@vger.kernel.org
[mailto:linux-kernel-owner@vger.kernel.org]On Behalf Of Gian-Yan Xu
Sent: Monday, October 15, 2001 5:12 PM
To: linux-kernel@vger.kernel.org
Subject: ptrace bug



Today I try to get register's value via ptrace(PTRACE_GETREGS, ...),
but only EBX, ECX, EDX, ESI, EDI, EBP, EAX registers are correct.
I notice that file /usr/src/linux/include/asm/ptrace.h:

#define FS 9
#define GS 10

but in the declare of struct pt_regs:

struct pt_regs {
long ebx;
long ecx;
long edx;
long esi;
long edi;
long ebp;
long eax;
int xds;
int xes;
long orig_eax;
long eip;
int xcs;
long eflags;
long esp;
int xss;
};

There is no xfs/xgs member in that struct, and the #define FRAME_SIZE 17
is not match the number of member in the pt_regs struct.


In addition, in the ptrace.c:

case PTRACE_GETREGS: { /* Get all gp regs from the child. */
if (!access_ok(VERIFY_WRITE, (unsigned *)data,
FRAME_SIZE*sizeo(long))) {
ret = -EIO;
break;
}
for ( i = 0; i < FRAME_SIZE*sizeof(long); i += sizeof(long) ) {
__put_user(getreg(child, i),(unsigned long *) data);
data += sizeof(long);
}
ret = 0;

FRAME_SIZE*sizeof(long) is larger than sizeof(struct pt_regs),
the ptrace() will overwrite the data of parent process!

To fix the bug, try this patch:

--- ptrace.h.orig Mon Oct 15 21:00:48 2001
+++ ptrace.h Mon Oct 15 21:05:56 2001
@@ -33,6 +33,8 @@
long eax;
int xds;
int xes;
+ int xfs;
+ int xgs;
long orig_eax;
long eip;
int xcs;


--
Best regards,
Gian-Yain Xu. (kids@linux.ee.tku.edu.tw)


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