[uml-devel] [PATCH] UML: Fix gdbserver "Couldn't write debug register" error

[uml-devel] [PATCH] UML: Fix gdbserver "Couldn't write debug register" error

[Barry Friedman]

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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