Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
"Jean Delvare" <[EMAIL PROTECTED]> said: [Sttributions missing, sorry] > > > Think about it. If the pointer could be NULL, then it's unlikely that > > > the bug would have gone unnoticed so far (unless the code is very > > > recent). Coverity found 3 such bugs in one i2c driver [1], and the > > > correct solution was to NOT check for NULL because it just couldn't > > > happen. > > No, there is a third case: the pointer can be NULL, but the compiler > > happened to move the dereference down to after the check. > Wow. Great point. I completely missed that possibility. In fact I didn't > know that the compiler could possibly alter the order of the > instructions. For one thing, I thought it was simply not allowed to. For > another, I didn't know that it had been made so aware that it could > actually figure out how to do this kind of things. What a mess. Let's > just hope that the gcc folks know their business :) The compiler is most definitely /not/ allowed to change the results the code gives. -- Dr. Horst H. von Brand User #22616 counter.li.org Departamento de Informatica Fono: +56 32 654431 Universidad Tecnica Federico Santa Maria +56 32 654239 Casilla 110-V, Valparaiso, ChileFax: +56 32 797513 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
On Mar 29, 2005, at 09:22, Daniel Jacobowitz wrote: The thing GCC is most likely to do with this code is discard the NULL check entirely and leave only the oops; the "if (!card)" can not be reached without passing through "card->amplifier", and a pointer which is dereferenced can not be NULL in a valid program. Not true! It is perfectly legal on a large number of platforms to explicitly mmap something at the address 0, at which point dereferencing a null pointer is exactly like dereferencing any other pointer on the heap. Doing so is not recommended except in a few special cases for emulators and such, but it works to some extent. Cheers, Kyle Moffett -BEGIN GEEK CODE BLOCK- Version: 3.12 GCM/CS/IT/U d- s++: a18 C>$ UB/L/X/*(+)>$ P+++()>$ L(+++) E W++(+) N+++(++) o? K? w--- O? M++ V? PS+() PE+(-) Y+ PGP+++ t+(+++) 5 X R? tv-(--) b(++) DI+ D+ G e->$ h!*()>++$ r !y?(-) --END GEEK CODE BLOCK-- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
On Mon, Mar 28, 2005 at 10:23:48PM -0800, Andrew Morton wrote: > > > -int old=card->amplifier; > > > +int old; > > > if(!card) > > > { > > > CS_DBGOUT(CS_ERROR, 2, printk(KERN_INFO > > > "cs46xx: amp_hercules() called before > > initialized.\n")); > > > return; > > > } > > > +old = card->amplifier; > No, there is a third case: the pointer can be NULL, but the compiler > happened to move the dereference down to after the check. > > If the optimiser is later changed, or if someone tries to compile the code > with -O0, it will oops. The thing GCC is most likely to do with this code is discard the NULL check entirely and leave only the oops; the "if (!card)" can not be reached without passing through "card->amplifier", and a pointer which is dereferenced can not be NULL in a valid program. -- Daniel Jacobowitz CodeSourcery, LLC - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Jean Delvare wrote: Wow. Great point. I completely missed that possibility. In fact I didn't know that the compiler could possibly alter the order of the instructions. For one thing, I thought it was simply not allowed to. For another, I didn't know that it had been made so aware that it could actually figure out how to do this kind of things. What a mess. Let's just hope that the gcc folks know their business :) I'll try to remember this next time I debug something. Do not assume that instructions are run in the order seen in the source. Irk. It gets better, in that the cpus themselves can reorder instructions. This becomes interesting when dealing with memory being shared between multiple cpus on SMP machines. Either you need to use existing locking primitives which enforce ordering or else you need to use explicit cpu-level ordering instructions to ensure that data gets written/read in the expected order. (See "mb" and friends in the kernel code.) Then you get into potential caching issues with memory mapped at different addresses on cpus with VIVT caches, and that introduces more issues. Computers are perfectly predictable, as long as you understand exactly what you've told them to do... Chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Hi Andrew, all, > > Think about it. If the pointer could be NULL, then it's unlikely that > > the bug would have gone unnoticed so far (unless the code is very > > recent). Coverity found 3 such bugs in one i2c driver [1], and the > > correct solution was to NOT check for NULL because it just couldn't > > happen. > > No, there is a third case: the pointer can be NULL, but the compiler > happened to move the dereference down to after the check. Wow. Great point. I completely missed that possibility. In fact I didn't know that the compiler could possibly alter the order of the instructions. For one thing, I thought it was simply not allowed to. For another, I didn't know that it had been made so aware that it could actually figure out how to do this kind of things. What a mess. Let's just hope that the gcc folks know their business :) I'll try to remember this next time I debug something. Do not assume that instructions are run in the order seen in the source. Irk. > If the optimiser is later changed, or if someone tries to compile the code > with -O0, it will oops. Interesting. Then wouldn't it be worth attempting such compilations at times, and see if we get additional oops? Doesn't gcc have a flag to specifically forbid this family of optimizations? Thanks, -- Jean Delvare - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Jean Delvare <[EMAIL PROTECTED]> wrote: > > > This patch fixes a check after use found by the Coverity checker. > > (...) > > static void amp_hercules(struct cs_card *card, int change) > > { > > - int old=card->amplifier; > > + int old; > >if(!card) > >{ > >CS_DBGOUT(CS_ERROR, 2, printk(KERN_INFO > >"cs46xx: amp_hercules() called before initialized.\n")); > >return; > >} > > + old = card->amplifier; > > I see that you are fixing many bugs like this one today, all reported by > Coverity. In all cases (as far as I could see at least) you are moving > the dereference after the check. Of course it prevents any NULL pointer > dereference, and will make Coverity happy. However, I doubt that this is > always the correct solution. > > Think about it. If the pointer could be NULL, then it's unlikely that > the bug would have gone unnoticed so far (unless the code is very > recent). Coverity found 3 such bugs in one i2c driver [1], and the > correct solution was to NOT check for NULL because it just couldn't > happen. No, there is a third case: the pointer can be NULL, but the compiler happened to move the dereference down to after the check. If the optimiser is later changed, or if someone tries to compile the code with -O0, it will oops. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
On Mon, 28 Mar 2005, L. A. Walsh wrote: > However, in this case, if the author is _certain_ the > pointer can never be NULL, than an "ASSERT(card!=NULL);" might > be appropriate, where ASSERT is a macro that normally compiles > in the check, but could compile to "nothing" for embedded or > kernels that aren't being developed in. If the kernel dereferences a NULL pointer, it produces an extensive bug report. The automatic oops is about the most useful thing to do if a pointer is unexpectedly NULL. -Daniel *This .sig left intentionally blank* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Adrian Bunk wrote: On Sun, Mar 27, 2005 at 11:21:58PM +0200, Jean Delvare wrote: There are two cases: 1. NULL is impossible, the check is superfluous 2. this was an actual bug In the first case, my patch doesn't do any harm (a superfluous isn't a real bug). In the second case, it fixed a bug. It might be a bug not many people hit because it might be in some error path of some esoteric driver. If a maintainer of a well-maintained subsystem like i2c says "The check is superfluous." that's the perfect solution. But in less maintained parts of the kernel, even a low possibility that it fixes a possible bug is IMHO worth making such a riskless patch. --- I'd agree in [al]most any part of the kernel. Unless it is extremely time critical code, subroutines should expect possible garbage from their callers. Just because it may be perfect today doesn't mean someone down the line won't call the routine with less than perfect parameters. It used to be called "defensive" programming. However, in this case, if the author is _certain_ the pointer can never be NULL, than an "ASSERT(card!=NULL);" might be appropriate, where ASSERT is a macro that normally compiles in the check, but could compile to "nothing" for embedded or kernels that aren't being developed in. -linda Thanks, Jean Delvare cu Adrian - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Jean Delvare schrieb: Hi Adrian, There are two cases: 1. NULL is impossible, the check is superfluous 2. this was an actual bug Agreed. In the first case, my patch doesn't do any harm (a superfluous isn't a real bug). The fact that it isn't a bug does not imply that the patch doesn't harm. Tricking the reader into thinking that something can happen when it in fact cannot does possibly harm in the long run. In the second case, it fixed a bug. It might be a bug not many people hit because it might be in some error path of some esoteric driver. True, except that e.g. the sg_mmap() function of scsi/sg hardly falls into this category. Same for fbcon_init() in video/console/fbcon. You don't seem to treat core code any differently than esoteric drivers. If a maintainer of a well-maintained subsystem like i2c says "The check is superfluous." that's the perfect solution. But in less maintained parts of the kernel, even a low possibility that it fixes a possible bug is IMHO worth making such a riskless patch. This is where my opinion strongly differs. The very fact that these "check after use" cases were not fixed yet means that they are not hit frequently, if ever, regardless of how popular the driver is. This means that we have (relatively) plenty of time to fix them. At least Coverity or a similar tool will keep reminding us to take a look and decide what must be done after we carefully checked the code. Your approach will not let us do that. Mass-posting these patches here is likely to make them end in Andrew's tree soon and to Linus' right after that is nobody objects, right? If you can make sure that none of these patches ever reaches Linus' tree without their respective driver maintainer having confirmed that they were the right thing to do, then fine with me, but it doesn't look like the way things will go. I think that you'd be better just telling the maintainers about the problem without providing an arbitrary patch, so that they will actually look into the problem with their advanced knowledge of the driver, rather than merely ack'ing that the patch looks OK, which I think is what will happen with your method. (I'd love to be proven wrong though.) Thanks, Hi! I fully disagree with you opinion. 1. Adrian sent this patches to the LKML _and_ to their maintainers. So the patches can be proved by 2 instances and not Adrian has to do this (of course he has to filter non-sense patches (I think he did so)). 2. I don't know in which case NULL pointers are usefull, but it seems they're. 3. Not only Maintainers have knowledge about the driver and can decide whether a patch is useful or not (this hierarchical structure is outdated (it's adapeted from the middle ages and a reason why many developers (like me) don't want to waste their time on fixing little Kernel bugs (in most cases you have to resend your patch several times to get it appllied))). This only elongates the development time and is not necessary because as mentioned above it's sent to the LKML and the maintainers have subscribed the LKML and can give their comment here and not in a private discussion. Because of this I recommend everybody to use such tools and sent such patches, increasing the quality of the source and fixing problems like the problems Adrians patches fix, to the LKML. Matthias-Christian Ott - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
On Mon, Mar 28, 2005 at 12:34:01AM +0200, Jean Delvare wrote: > I think that you'd be better just telling the > maintainers about the problem without providing an arbitrary patch, so > that they will actually look into the problem with their advanced > knowledge of the driver FWIW, I agree with Jean. Applying these patches without adequate review is like applying __iomem or __user fixes where they've been generated with the aim of shutting up sparse, rather than considering whether the warning is actually valid. Once the warning is gone, the real problem is lost and never gets looked at. -- Russell King Linux kernel2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Hi Adrian, > There are two cases: > 1. NULL is impossible, the check is superfluous > 2. this was an actual bug Agreed. > In the first case, my patch doesn't do any harm (a superfluous isn't > a real bug). The fact that it isn't a bug does not imply that the patch doesn't harm. Tricking the reader into thinking that something can happen when it in fact cannot does possibly harm in the long run. > In the second case, it fixed a bug. > It might be a bug not many people hit because it might be in some > error path of some esoteric driver. True, except that e.g. the sg_mmap() function of scsi/sg hardly falls into this category. Same for fbcon_init() in video/console/fbcon. You don't seem to treat core code any differently than esoteric drivers. > If a maintainer of a well-maintained subsystem like i2c says > "The check is superfluous." that's the perfect solution. > > But in less maintained parts of the kernel, even a low possibility > that it fixes a possible bug is IMHO worth making such a riskless > patch. This is where my opinion strongly differs. The very fact that these "check after use" cases were not fixed yet means that they are not hit frequently, if ever, regardless of how popular the driver is. This means that we have (relatively) plenty of time to fix them. At least Coverity or a similar tool will keep reminding us to take a look and decide what must be done after we carefully checked the code. Your approach will not let us do that. Mass-posting these patches here is likely to make them end in Andrew's tree soon and to Linus' right after that is nobody objects, right? If you can make sure that none of these patches ever reaches Linus' tree without their respective driver maintainer having confirmed that they were the right thing to do, then fine with me, but it doesn't look like the way things will go. I think that you'd be better just telling the maintainers about the problem without providing an arbitrary patch, so that they will actually look into the problem with their advanced knowledge of the driver, rather than merely ack'ing that the patch looks OK, which I think is what will happen with your method. (I'd love to be proven wrong though.) Thanks, -- Jean Delvare - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
On Sun, Mar 27, 2005 at 11:21:58PM +0200, Jean Delvare wrote: > Hi Adrian, > > > This patch fixes a check after use found by the Coverity checker. > > (...) > > static void amp_hercules(struct cs_card *card, int change) > > { > > - int old=card->amplifier; > > + int old; > > if(!card) > > { > > CS_DBGOUT(CS_ERROR, 2, printk(KERN_INFO > > "cs46xx: amp_hercules() called before initialized.\n")); > > return; > > } > > + old = card->amplifier; > > I see that you are fixing many bugs like this one today, all reported by > Coverity. In all cases (as far as I could see at least) you are moving > the dereference after the check. Of course it prevents any NULL pointer > dereference, and will make Coverity happy. However, I doubt that this is > always the correct solution. > > Think about it. If the pointer could be NULL, then it's unlikely that > the bug would have gone unnoticed so far (unless the code is very > recent). Coverity found 3 such bugs in one i2c driver [1], and the > correct solution was to NOT check for NULL because it just couldn't > happen. Could be the case for all the bugs you are fixing right now too. > > [1] http://linux.bkbits.net:8080/linux-2.5/[EMAIL PROTECTED] > > Coverity and similar tools are a true opportunity for us to find out and > study suspect parts of our code. Please do not misuse these tools! The > goal is NOT to make the tools happy next time you run them, but to > actually fix the problems, once and for all. If you focus too much on > fixing the problems quickly rather than fixing them cleanly, then we > forever lose the opportunity to clean our code, because the problems > will then be hidden. If you look at case [1] above, you'll see that we > were able to fix more than just what Coverity had reported. >... There are two cases: 1. NULL is impossible, the check is superfluous 2. this was an actual bug In the first case, my patch doesn't do any harm (a superfluous isn't a real bug). In the second case, it fixed a bug. It might be a bug not many people hit because it might be in some error path of some esoteric driver. If a maintainer of a well-maintained subsystem like i2c says "The check is superfluous." that's the perfect solution. But in less maintained parts of the kernel, even a low possibility that it fixes a possible bug is IMHO worth making such a riskless patch. > Thanks, > Jean Delvare cu Adrian -- "Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Do not misuse Coverity please (Was: sound/oss/cs46xx.c: fix a check after use)
Hi Adrian, > This patch fixes a check after use found by the Coverity checker. > (...) > static void amp_hercules(struct cs_card *card, int change) > { > - int old=card->amplifier; > + int old; > if(!card) > { > CS_DBGOUT(CS_ERROR, 2, printk(KERN_INFO > "cs46xx: amp_hercules() called before initialized.\n")); > return; > } > + old = card->amplifier; I see that you are fixing many bugs like this one today, all reported by Coverity. In all cases (as far as I could see at least) you are moving the dereference after the check. Of course it prevents any NULL pointer dereference, and will make Coverity happy. However, I doubt that this is always the correct solution. Think about it. If the pointer could be NULL, then it's unlikely that the bug would have gone unnoticed so far (unless the code is very recent). Coverity found 3 such bugs in one i2c driver [1], and the correct solution was to NOT check for NULL because it just couldn't happen. Could be the case for all the bugs you are fixing right now too. [1] http://linux.bkbits.net:8080/linux-2.5/[EMAIL PROTECTED] Coverity and similar tools are a true opportunity for us to find out and study suspect parts of our code. Please do not misuse these tools! The goal is NOT to make the tools happy next time you run them, but to actually fix the problems, once and for all. If you focus too much on fixing the problems quickly rather than fixing them cleanly, then we forever lose the opportunity to clean our code, because the problems will then be hidden. If you look at case [1] above, you'll see that we were able to fix more than just what Coverity had reported. Considering the very important flow of patches you are sending these days, I have to admit I am quite suspicious that you don't really investigate all issues individually as you should, but merely want to fix as many bugs as possible in a short amount of time. This is not, IMVHO, what needs to be done. Of course, if you actually investigated all these bugs and are certain that none of these checks can be removed because pointers can actually be NULL in regular cases, then please accept my humble apologizes and keep up with the good work. Thanks, -- Jean Delvare - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/