I was trying to run gdbserver in a UML instance and got "Couldn't
write debug register" after setting a breakpoint and continuing after
attaching from a remote gdb on the host. This is on an AMD x86_64
machine. The error message was resulting from getting EIO error when
making a PTRACE_POKEUSER call. I debugged the ptrace call handling on
the kernel side and found that the poke_user() and peek_user() calls
in arch/um/sys-x86_64/ptrace.c assume that long is 4 bytes instead of
8. With the patch below, I found that gdbserver works fine.
Thanks,
Barry
*** linux-2.6.32/arch/um/sys-x86_64/ptrace.c Wed Dec 2 19:51:21 2009
--- ../linux-2.6.32/arch/um/sys-x86_64/ptrace.c Wed Dec 9 10:54:36 2009
*************** int poke_user(struct task_struct *child,
*** 67,77 ****
if (addr < MAX_REG_OFFSET)
return putreg(child, addr, data);
else if ((addr >= offsetof(struct user, u_debugreg[0])) &&
(addr <= offsetof(struct user, u_debugreg[7]))) {
addr -= offsetof(struct user, u_debugreg[0]);
! addr = addr >> 2;
if ((addr == 4) || (addr == 5))
return -EIO;
child->thread.arch.debugregs[addr] = data;
return 0;
}
--- 67,77 ----
if (addr < MAX_REG_OFFSET)
return putreg(child, addr, data);
else if ((addr >= offsetof(struct user, u_debugreg[0])) &&
(addr <= offsetof(struct user, u_debugreg[7]))) {
addr -= offsetof(struct user, u_debugreg[0]);
! addr = addr >> 3; // long is 8 bytes on x86_64
if ((addr == 4) || (addr == 5))
return -EIO;
child->thread.arch.debugregs[addr] = data;
return 0;
}
*************** int peek_user(struct task_struct *child,
*** 112,122 ****
if (addr < MAX_REG_OFFSET)
tmp = getreg(child, addr);
else if ((addr >= offsetof(struct user, u_debugreg[0])) &&
(addr <= offsetof(struct user, u_debugreg[7]))) {
addr -= offsetof(struct user, u_debugreg[0]);
! addr = addr >> 2;
tmp = child->thread.arch.debugregs[addr];
}
return put_user(tmp, (unsigned long *) data);
}
--- 112,122 ----
if (addr < MAX_REG_OFFSET)
tmp = getreg(child, addr);
else if ((addr >= offsetof(struct user, u_debugreg[0])) &&
(addr <= offsetof(struct user, u_debugreg[7]))) {
addr -= offsetof(struct user, u_debugreg[0]);
! addr = addr >> 3; // long is 8 bytes on x86_64
tmp = child->thread.arch.debugregs[addr];
}
return put_user(tmp, (unsigned long *) data);
}
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