On Mon, 2021-03-01 at 12:17 +0100, Paolo Bonzini wrote:
> If kvm_arch_remove_sw_breakpoint finds that a software breakpoint does not
> have an INT3 instruction, it fails. This can happen if one sets a
> software breakpoint in a kernel module and then reloads it. gdb then
> thinks the breakpoint cannot be deleted and there is no way to add it
> back.
>
> Suggested-by: Maxim Levitsky <mlevi...@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> target/i386/kvm/kvm.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 0b5755e42b..c8d61daf68 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -4352,8 +4352,13 @@ int kvm_arch_remove_sw_breakpoint(CPUState *cs, struct
> kvm_sw_breakpoint *bp)
> {
> uint8_t int3;
>
> - if (cpu_memory_rw_debug(cs, bp->pc, &int3, 1, 0) || int3 != 0xcc ||
> - cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 1, 1)) {
> + if (cpu_memory_rw_debug(cs, bp->pc, &int3, 1, 0)) {
> + return -EINVAL;
> + }
> + if (int3 != 0xcc) {
> + return 0;
> + }
> + if (cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 1, 1)) {
> return -EINVAL;
> }
> return 0;
There still remains a philosopical question if kvm_arch_remove_sw_breakpoint
should always return 0, since for the usual case of kernel debugging where
a breakpoint is in unloaded module, the above will probably still fail
as paging for this module is removed as well by the kernel.
It is still better though so:
Reviewed-by: Maxim Levitsky <mlevi...@redhat.com>
Note that I managed to make lx-symbols to work in a very stable way
with attached WIP patch I hacked on this Sunday.
I will send a cleaned up version of it to upstream when I have time.
Since I make gdb unload the symbols, it works even without this patch.
Added Stefano Garzarella to CC as I know that he tried to make this work as
well.
https://lkml.org/lkml/2020/10/5/514
Best regards,
Maxim Levitsky
commit 86f0d5a08a40528350589ed54126f06d4ac293a8
Author: Maxim Levitsky <mlevi...@redhat.com>
Date: Sun Feb 28 23:52:08 2021 +0200
gdb: rework gdb debug script
Signed-off-by: Maxim Levitsky <mlevi...@redhat.com>
diff --git a/scripts/gdb/linux/symbols.py b/scripts/gdb/linux/symbols.py
index 1be9763cf8bb..13f21afc2059 100644
--- a/scripts/gdb/linux/symbols.py
+++ b/scripts/gdb/linux/symbols.py
@@ -17,6 +17,24 @@ import re
from linux import modules, utils
+def save_state():
+ breakpoints = []
+ if hasattr(gdb, 'breakpoints') and not gdb.breakpoints() is None:
+ for bp in gdb.breakpoints():
+ breakpoints.append({'breakpoint': bp, 'enabled': bp.enabled})
+ bp.enabled = False
+
+ show_pagination = gdb.execute("show pagination", to_string=True)
+ pagination = show_pagination.endswith("on.\n")
+ gdb.execute("set pagination off")
+
+ return {"breakpoints":breakpoints, "show_pagination": show_pagination}
+
+def load_state(state):
+ for breakpoint in state["breakpoints"]:
+ breakpoint['breakpoint'].enabled = breakpoint['enabled']
+ gdb.execute("set pagination %s" % ("on" if state["show_pagination"] else "off"))
+
if hasattr(gdb, 'Breakpoint'):
class LoadModuleBreakpoint(gdb.Breakpoint):
@@ -30,26 +48,38 @@ if hasattr(gdb, 'Breakpoint'):
module_name = module['name'].string()
cmd = self.gdb_command
+ # module already loaded, false alarm
+ if module_name in cmd.loaded_modules:
+ return False
+
# enforce update if object file is not found
cmd.module_files_updated = False
# Disable pagination while reporting symbol (re-)loading.
# The console input is blocked in this context so that we would
# get stuck waiting for the user to acknowledge paged output.
- show_pagination = gdb.execute("show pagination", to_string=True)
- pagination = show_pagination.endswith("on.\n")
- gdb.execute("set pagination off")
+ state = save_state()
+ cmd.load_module_symbols(module)
+ load_state(state)
+ return False
- if module_name in cmd.loaded_modules:
- gdb.write("refreshing all symbols to reload module "
- "'{0}'\n".format(module_name))
- cmd.load_all_symbols()
- else:
- cmd.load_module_symbols(module)
+ class UnLoadModuleBreakpoint(gdb.Breakpoint):
+ def __init__(self, spec, gdb_command):
+ super(UnLoadModuleBreakpoint, self).__init__(spec, internal=True)
+ self.silent = True
+ self.gdb_command = gdb_command
- # restore pagination state
- gdb.execute("set pagination %s" % ("on" if pagination else "off"))
+ def stop(self):
+ module = gdb.parse_and_eval("mod")
+ module_name = module['name'].string()
+ cmd = self.gdb_command
+ if not module_name in cmd.loaded_modules:
+ return False
+
+ state = save_state()
+ cmd.unload_module_symbols(module)
+ load_state(state)
return False
@@ -64,8 +94,9 @@ lx-symbols command."""
module_paths = []
module_files = []
module_files_updated = False
- loaded_modules = []
+ loaded_modules = {}
breakpoint = None
+ ubreakpoint = None
def __init__(self):
super(LxSymbols, self).__init__("lx-symbols", gdb.COMMAND_FILES,
@@ -129,21 +160,32 @@ lx-symbols command."""
filename=module_file,
addr=module_addr,
sections=self._section_arguments(module))
+
gdb.execute(cmdline, to_string=True)
- if module_name not in self.loaded_modules:
- self.loaded_modules.append(module_name)
+ self.loaded_modules[module_name] = {"module_file": module_file,
+ "module_addr": module_addr}
else:
gdb.write("no module object found for '{0}'\n".format(module_name))
+ def unload_module_symbols(self, module):
+ module_name = module['name'].string()
+
+ module_file = self.loaded_modules[module_name]["module_file"]
+ module_addr = self.loaded_modules[module_name]["module_addr"]
+
+ gdb.write("unloading @{addr}: {filename}\n".format(
+ addr=module_addr, filename=module_file))
+ cmdline = "remove-symbol-file {filename}".format(
+ filename=module_file)
+
+ gdb.execute(cmdline, to_string=True)
+ del self.loaded_modules[module_name]
+
+
def load_all_symbols(self):
gdb.write("loading vmlinux\n")
- # Dropping symbols will disable all breakpoints. So save their states
- # and restore them afterward.
- saved_states = []
- if hasattr(gdb, 'breakpoints') and not gdb.breakpoints() is None:
- for bp in gdb.breakpoints():
- saved_states.append({'breakpoint': bp, 'enabled': bp.enabled})
+ state = save_state()
# drop all current symbols and reload vmlinux
orig_vmlinux = 'vmlinux'
@@ -153,15 +195,14 @@ lx-symbols command."""
gdb.execute("symbol-file", to_string=True)
gdb.execute("symbol-file {0}".format(orig_vmlinux))
- self.loaded_modules = []
+ self.loaded_modules = {}
module_list = modules.module_list()
if not module_list:
gdb.write("no modules found\n")
else:
[self.load_module_symbols(module) for module in module_list]
- for saved_state in saved_states:
- saved_state['breakpoint'].enabled = saved_state['enabled']
+ load_state(state)
def invoke(self, arg, from_tty):
self.module_paths = [os.path.expanduser(p) for p in arg.split()]
@@ -177,8 +218,13 @@ lx-symbols command."""
if self.breakpoint is not None:
self.breakpoint.delete()
self.breakpoint = None
- self.breakpoint = LoadModuleBreakpoint(
- "kernel/module.c:do_init_module", self)
+ self.breakpoint = LoadModuleBreakpoint("kernel/module.c:do_init_module", self)
+
+ if self.ubreakpoint is not None:
+ self.ubreakpoint.delete()
+ self.ubreakpoint = None
+ self.ubreakpoint = UnLoadModuleBreakpoint("kernel/module.c:free_module", self)
+
else:
gdb.write("Note: symbol update on module loading not supported "
"with this gdb version\n")
