Running gdbserver in a UML instance displays "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. Fix here calculates the register index based on 8 byte long.
arch/um/sys-x86_64/ptrace.c
- poke_user(): Fix register index calc based on 8 byte long
- peek_user(): Fix register index calc based on 8 byte long
---
arch/um/sys-x86_64/ptrace.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/um/sys-x86_64/ptrace.c b/arch/um/sys-x86_64/ptrace.c
index f3458d7..e491e48 100644
--- a/arch/um/sys-x86_64/ptrace.c
+++ b/arch/um/sys-x86_64/ptrace.c
@@ -69,7 +69,7 @@ int poke_user(struct task_struct *child, long addr, long 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;
+ addr = addr >> 3;
if ((addr == 4) || (addr == 5))
return -EIO;
child->thread.arch.debugregs[addr] = data;
@@ -114,7 +114,7 @@ int peek_user(struct task_struct *child, long addr, long
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;
+ addr = addr >> 3;
tmp = child->thread.arch.debugregs[addr];
}
return put_user(tmp, (unsigned long *) data);
--
1.7.0.1
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