From: Steven Rostedt While debugging a module, I found that unloading the module and then reading the ring buffer can cause strange side effects, including a kernel crash. This is due to the trace_bprintk(). The trace_bprintk() is a faster version of trace_printk(). The difference is that trace_bprintk() only copies the arguments and a pointer to the format string into the ring buffer. If a module uses this function and is unloaded, the pointer back to the format string in the module is still around. If the trace file is read, then the pointer is referenced and this can cause a kernel oops. The simple solution is to not let modules use trace_bprintk() and instead it will use the slower version of this. When talking with Frederic Weisbecker about it, he suggested not to punish modules that can not be unloaded since they do not have this side effect. Modules that can not be unloaded can still use trace_bprintk(). We added a check for MODVERSIONS to be set to make sure that the module and kernel have the same options. If you run without MODVERSIONS set, and you load a module that was compiled differently, then that's just your tough luck. Cc: Frederic Weisbecker Cc: Thomas Gleixner Signed-off-by: Steven Rostedt --- include/linux/kernel.h | 21 +++++++++++++++++++-- kernel/trace/trace_printk.c | 2 ++ 2 files changed, 21 insertions(+), 2 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 2b0a35e..1003476 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -538,6 +538,23 @@ do { \ ____trace_printk_check_format(fmt, ##args); \ } while (0) +/* + * Module code must not use trace_bprintk, because if it is unloaded + * then we leave a pointer back to the module code inside + * the ring buffer, and then reading the ring buffer may cause a bug. + * + * We do allow for modules to use it if the kernel does not allow + * unloading of modules, and MODVERSIONS is set (to make sure kernel + * and module are the same). If you load modules without MODVERSIONS + * set, then you deserve what you get. + */ +#if defined(MODULE) && \ + (defined(CONFIG_MODULE_UNLOAD) || !defined(CONFIG_MODVERSIONS)) +# define FORCE_TRACEPRINTK 1 +#else +# define FORCE_TRACEPRINTK 0 +#endif + /** * trace_printk - printf formatting in the ftrace buffer * @fmt: the printf format for printing @@ -558,14 +575,14 @@ do { \ #define trace_printk(fmt, args...) \ do { \ __trace_printk_check_format(fmt, ##args); \ - if (__builtin_constant_p(fmt)) { \ + if (__builtin_constant_p(fmt) && !FORCE_TRACEPRINTK) { \ static const char *trace_printk_fmt \ __attribute__((section("__trace_printk_fmt"))) = \ __builtin_constant_p(fmt) ? fmt : NULL; \ \ __trace_bprintk(_THIS_IP_, trace_printk_fmt, ##args); \ } else \ - __trace_printk(_THIS_IP_, fmt, ##args); \ + __trace_printk(_THIS_IP_, fmt, ##args); \ } while (0) extern int diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c index 2547d88..c4a5db6 100644 --- a/kernel/trace/trace_printk.c +++ b/kernel/trace/trace_printk.c @@ -115,7 +115,9 @@ int __trace_bprintk(unsigned long ip, const char *fmt, ...) va_end(ap); return ret; } +#if !FORCE_TRACEPRINTK EXPORT_SYMBOL_GPL(__trace_bprintk); +#endif int __ftrace_vbprintk(unsigned long ip, const char *fmt, va_list ap) { -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/