Messages in this thread | | | Date | Mon, 19 Sep 2011 17:14:23 -0700 | From | Tim Bird <> | Subject | kdb - can't set breakpoint by default on x86 |
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I recently tried using KDB on a 3.0.1 kernel, on an x86 target, using a fairly-close-to-default kernel configuration.
I found that I could not set a breakpoint. It sets inside KDB, but then fails to apply the breakpoint when resuming to user space:
-------------------------------- /proc # echo g >sysrq-trigger [2689816.806249] SysRq : DEBUG
Entering kdb (current=0xc7661d40, pid 511) on processor 0 due to Keyboard Entry
[0]kdb> bp sys_sync Instruction(i) BP #0 at 0xc111ce70 (sys_sync) is enabled addr at 00000000c111ce70, hardtype=0 installed=0
[0]kdb> g _kdb_bp_install: failed to set breakpoint at 0xc111ce70 /proc # sync /proc # --------------------------------
I discovered that this was due to the text segment being marked read-only, by mark_rodata_ro() in arch/x86/mm/init_32.c
This is controlled by CONFIG_DEBUG_RODATA. With CONFIG_DEBUG_RODATA=y, I get the following message on startup:
Write protecting the kernel read-only data: 6245
Also, breakpoints don't work.
Turning this off solves the problem, but seems counter-intuitive. CONFIG_DEBUG_RODATA depends on CONFIG_DEBUG_KERNEL.
Should I: 1) turn off CONFIG_DEBUG_RODATA automatically when CONFIG_KGDB_KDB is set? or 2) make CONFIG_DEBUG_RODATA depend on !KGDB_KDB? or 3) something else more subtle?
Thanks, -- Tim
============================= Tim Bird Architecture Group Chair, CE Workgroup of the Linux Foundation Senior Staff Engineer, Sony Network Entertainment =============================
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