From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751419AbXAPXfh (ORCPT ); Tue, 16 Jan 2007 18:35:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751791AbXAPXfh (ORCPT ); Tue, 16 Jan 2007 18:35:37 -0500 Received: from pentafluge.infradead.org ([213.146.154.40]:56215 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751419AbXAPXfg (ORCPT ); Tue, 16 Jan 2007 18:35:36 -0500 Date: Tue, 16 Jan 2007 23:35:28 +0000 From: Christoph Hellwig To: Alan Stern Cc: Andrew Morton , Prasanna S Panchamukhi , Kernel development list Subject: Re: [PATCH] Kwatch: kernel watchpoints using CPU debug registers Message-ID: <20070116233528.GA19834@infradead.org> Mail-Followup-To: Christoph Hellwig , Alan Stern , Andrew Morton , Prasanna S Panchamukhi , Kernel development list References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.2i X-SRS-Rewrite: SMTP reverse-path rewritten from by pentafluge.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Fir4st I'd say thanks a lot for forward-porting this, it's really useful feature for all kinds of nasty debugging. I think you should split this into two patches, one for the debugreg infrastructure, and one for the actual kwatch code. Also I think you provide one (or even a few) example wathes for trivial things, say updating i_ino for an inode given through debugfs. Some comments on the code below: > --- /dev/null > +++ usb-2.6/arch/i386/kernel/debugreg.c > @@ -0,0 +1,182 @@ > +/* > + * Debug register > + * arch/i386/kernel/debugreg.c Please don't put in comments that mention the name of the containing file. Also the "Debug register" comments seems rather useless. > + * 2002-Oct Created by Vamsi Krishna S and > + * Bharata Rao to provide debug register > + * allocation mechanism. > + * 2004-Oct Updated by Prasanna S Panchamukhi with > + * idr_allocations mechanism as suggested by Andi Kleen. I think these kinds of comments aren't in fashion anymore either, all changelogs should be in git commit messages and initial credits go into the first commit message. > +struct debugreg dr_list[DR_MAX]; > +static spinlock_t dr_lock = SPIN_LOCK_UNLOCKED; I think you're supposed to use magic DEFINE_SPINLOCK macro these days. > +unsigned long dr7_global_mask = DR_CONTROL_RESERVED | DR_GLOBAL_SLOWDOWN | > + DR_GLOBAL_ENABLE_MASK; I'd rahter keep this static and make set_process_dr7 a non-inline function. > + > +static unsigned long dr7_global_reg_mask(unsigned int regnum) > +{ > + return (0xf << (16 + regnum * 4)) | (0x1 << (regnum * 2)); > +} > + > +static int get_dr(int regnum, int flag) > +{ > + if (flag == DR_ALLOC_GLOBAL && !dr_list[regnum].flag) { > + dr_list[regnum].flag = flag; > + dr7_global_mask |= dr7_global_reg_mask(regnum); > + return regnum; > + } > + if (flag == DR_ALLOC_LOCAL && > + dr_list[regnum].flag != DR_ALLOC_GLOBAL) { > + dr_list[regnum].flag = flag; > + dr_list[regnum].use_count++; > + return regnum; > + } > + return -1; This looks rather poorly structured, as the function does compltely different things depending on the flags passed in. > +static void free_dr(int regnum) > +{ > + if (dr_list[regnum].flag == DR_ALLOC_LOCAL) { > + if (!--dr_list[regnum].use_count) > + dr_list[regnum].flag = 0; > + } else { > + dr_list[regnum].flag = 0; > + dr_list[regnum].use_count = 0; > + dr7_global_mask &= ~(dr7_global_reg_mask(regnum)); > + } > +} Same here. > +int dr_alloc(int regnum, int flag) > +{ > + int ret = -1; > + > + spin_lock(&dr_lock); > + if (regnum >= 0 && regnum < DR_MAX) > + ret = get_dr(regnum, flag); > + else if (regnum == DR_ANY) { > + > + /* gdb allocates local debug registers starting from 0. > + * To help avoid conflicts, we'll start from the other end. > + */ > + for (regnum = DR_MAX - 1; regnum >= 0; --regnum) { > + ret = get_dr(regnum, flag); > + if (ret >= 0) > + break; > + } > + } else > + printk(KERN_ERR "dr_alloc: " > + "Cannot allocate debug register %d\n", regnum); > + spin_unlock(&dr_lock); > + return ret; I suspect this should be replaced wit ha global and local variant to fix the above mentioned issue. It's a tiny bit duplicated code, but seems much cleaner. > +static int get_dr(int regnum, int flag) > +{ > + if (flag == DR_ALLOC_GLOBAL && !dr_list[regnum].flag) { > + dr_list[regnum].flag = flag; > + dr7_global_mask |= dr7_global_reg_mask(regnum); > + return regnum; > + } > + if (flag == DR_ALLOC_LOCAL && > + dr_list[regnum].flag != DR_ALLOC_GLOBAL) { > + dr_list[regnum].flag = flag; > + dr_list[regnum].use_count++; > + return regnum; > + } > + return -1; Same comments about global vs local here. > + > +EXPORT_SYMBOL(dr_alloc); > +EXPORT_SYMBOL(dr_free); I don't think we want these exported at all, and if a proper modular user shows up they should be _GPL as they're fairly lowlevel. Btw, the naming in the whole debugregs code should be consolidated to be debugreg_ instead of all kinds of different variants. > +#ifdef CONFIG_KWATCH > + > +/* Set the type, len and global flag in dr7 for a debug register */ > +#define SET_DR7(dr, regnum, type, len) do { \ > + dr &= ~(0xf << (16 + (regnum) * 4)); \ > + dr |= (((((len) - 1) << 2) | (type)) << \ > + (16 + (regnum) * 4)) | \ > + (0x2 << ((regnum) * 2)); \ > + } while (0) > + > +/* Disable a debug register by clearing the global/local flag in dr7 */ > +#define RESET_DR7(dr, regnum) dr &= ~(0x3 << ((regnum) * 2)) I don't think there's any point in making these macros conditional. Then again is there a good reason to mke these macros? > + * Kernel Watchpoint interface. > + * arch/i386/kernel/kwatch.c > + * > + * > + * 2002-Oct Created by Vamsi Krishna S for > + * Kernel Watchpoint implementation. > + * 2004-Oct Updated by Prasanna S Panchamukhi to > + * to make use of notifiers. > + */ Same comments about these comments applies as in debugreg.c > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include I think this should be linux/bitops.h these days. > + > +#define RF_MASK 0x00010000 > + > +static struct kwatch kwatch_list[DR_MAX]; > +static spinlock_t kwatch_lock = SPIN_LOCK_UNLOCKED; > +static unsigned long kwatch_in_progress; /* currently being handled */ Give that this is a bitmap the comment is rather misleading, it should probably be: /* * Bitmap of registers beeing handled. */ > +static void write_dr(int debugreg, unsigned long addr) > +{ > + switch (debugreg) { > + case 0: set_debugreg(addr, 0); break; > + case 1: set_debugreg(addr, 1); break; > + case 2: set_debugreg(addr, 2); break; > + case 3: set_debugreg(addr, 3); break; > + case 6: set_debugreg(addr, 6); break; > + case 7: set_debugreg(addr, 7); break; > + } > +} What's the point of this wrapper? > + > +#define write_dr7(val) set_debugreg((val), 7) > +#define read_dr7(val) get_debugreg((val), 7) And these? > + if (kwatch_in_progress) > + goto recursed; > + I don't think there's any point in this goto, just handle it inside the if block > + set_bit(debugreg, &kwatch_in_progress); > + > + spin_lock(&kwatch_lock); > + if ((unsigned long) kwatch_list[debugreg].addr != addr) > + goto out; > + > + if (kwatch_list[debugreg].handler) > + kwatch_list[debugreg].handler(&kwatch_list[debugreg], regs); > + > + if (kwatch_list[debugreg].type == DR_TYPE_EXECUTE) > + regs->eflags |= RF_MASK; > + out: Again, I think the goto here could be avoided and actually make the code cleanere. Also a local variable for kwatch_list[debugreg] with a short would probably make this section of code a lot more readable. > + > +static int __init init_kwatch(void) > +{ > + int err = 0; > + > + err = register_die_notifier(&kwatch_exceptions_nb); > + return err; > +} Just remove the err local variable here. > +EXPORT_SYMBOL_GPL(register_kwatch); > +EXPORT_SYMBOL_GPL(unregister_kwatch); Please move these exports close to the actual function definition. > --- /dev/null > +++ usb-2.6/include/asm-i386/kwatch.h > @@ -0,0 +1,60 @@ > +#ifndef _ASM_KWATCH_H > +#define _ASM_KWATCH_H > +/* > + * Kernel Watchpoint interface. > + * include/asm-i386/kwatch.h > + * 2002-Oct Created by Vamsi Krishna S for > + * Kernel Watchpoint implementation. > + */ Same comments once again. > +#include > +#include > + > +struct kwatch; > +typedef void (*kwatch_handler_t) (struct kwatch *, struct pt_regs *); > + > +struct kwatch { > + void *addr; /* location of watchpoint */ > + u8 length; /* range of address */ > + u8 type; /* type of watchpoint */ > + kwatch_handler_t handler; > +}; > + > +#define DR_TYPE_EXECUTE 0x0 /* Watchpoint types */ > +#define DR_TYPE_WRITE 0x1 > +#define DR_TYPE_IO 0x2 > +#define DR_TYPE_RW 0x3 I think large parts of this header should go into a new linux/kwatch.h so that generic code can use kwatches. > +config KWATCH > + bool "Kwatch points (EXPERIMENTAL)" > + depends on EXPERIMENTAL > + help > + Kwatch enables kernel-space data watchpoints using the processor's > + debug registers. It can be very useful for kernel debugging. > + If in doubt, say "N". I think we want different options for debugregs and kwatch. The debugreg one probably doesn't have to be actually user-visible, though.