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diff --git a/results/classifier/003/other/23300761 b/results/classifier/003/other/23300761 new file mode 100644 index 000000000..aaa3a8f86 --- /dev/null +++ b/results/classifier/003/other/23300761 @@ -0,0 +1,316 @@ +other: 0.963 +semantic: 0.950 +boot: 0.929 +instruction: 0.929 +KVM: 0.897 +network: 0.879 +mistranslation: 0.770 + +[Qemu-devel] [BUG] 216 Alerts reported by LGTM for QEMU (some might be release critical) + +Hi, +LGTM reports 16 errors, 81 warnings and 119 recommendations: +https://lgtm.com/projects/g/qemu/qemu/alerts/?mode=list +. +Some of them are already know (wrong format strings), others look like +real errors: +- several multiplication results which don't work as they should in +contrib/vhost-user-gpu, block/* (m->nb_clusters * s->cluster_size only +32 bit!), target/i386/translate.c and other files +- potential buffer overflows in gdbstub.c and other files +I am afraid that the overflows in the block code are release critical, +maybe that in target/i386/translate.c and other errors, too. +About half of the alerts are issues which can be fixed later. + +Regards + +Stefan + +On 13/07/19 19:46, Stefan Weil wrote: +> +> +LGTM reports 16 errors, 81 warnings and 119 recommendations: +> +https://lgtm.com/projects/g/qemu/qemu/alerts/?mode=list +. +> +> +Some of them are already know (wrong format strings), others look like +> +real errors: +> +> +- several multiplication results which don't work as they should in +> +contrib/vhost-user-gpu, block/* (m->nb_clusters * s->cluster_size only +> +32 bit!), target/i386/translate.c and other files +m->nb_clusters here is limited by s->l2_slice_size (see for example +handle_alloc) so I wouldn't be surprised if this is a false positive. I +couldn't find this particular multiplication in Coverity, but it has +about 250 issues marked as intentional or false positive so there's +probably a lot of overlap with what LGTM found. + +Paolo + +Am 13.07.2019 um 21:42 schrieb Paolo Bonzini: +> +On 13/07/19 19:46, Stefan Weil wrote: +> +> LGTM reports 16 errors, 81 warnings and 119 recommendations: +> +> +https://lgtm.com/projects/g/qemu/qemu/alerts/?mode=list +. +> +> +> +> Some of them are already known (wrong format strings), others look like +> +> real errors: +> +> +> +> - several multiplication results which don't work as they should in +> +> contrib/vhost-user-gpu, block/* (m->nb_clusters * s->cluster_size only +> +> 32 bit!), target/i386/translate.c and other files +> +m->nb_clusters here is limited by s->l2_slice_size (see for example +> +handle_alloc) so I wouldn't be surprised if this is a false positive. I +> +couldn't find this particular multiplication in Coverity, but it has +> +about 250 issues marked as intentional or false positive so there's +> +probably a lot of overlap with what LGTM found. +> +> +Paolo +> +From other projects I know that there is a certain overlap between the +results from Coverity Scan an LGTM, but it is good to have both +analyzers, and the results from LGTM are typically quite reliable. + +Even if we know that there is no multiplication overflow, the code could +be modified. Either the assigned value should use the same data type as +the factors (possible when there is never an overflow, avoids a size +extension), or the multiplication could use the larger data type by +adding a type cast to one of the factors (then an overflow cannot +happen, static code analysers and human reviewers have an easier job, +but the multiplication costs more time). + +Stefan + +Am 14.07.2019 um 15:28 hat Stefan Weil geschrieben: +> +Am 13.07.2019 um 21:42 schrieb Paolo Bonzini: +> +> On 13/07/19 19:46, Stefan Weil wrote: +> +>> LGTM reports 16 errors, 81 warnings and 119 recommendations: +> +>> +https://lgtm.com/projects/g/qemu/qemu/alerts/?mode=list +. +> +>> +> +>> Some of them are already known (wrong format strings), others look like +> +>> real errors: +> +>> +> +>> - several multiplication results which don't work as they should in +> +>> contrib/vhost-user-gpu, block/* (m->nb_clusters * s->cluster_size only +> +>> 32 bit!), target/i386/translate.c and other files +Request sizes are limited to 32 bit in the generic block layer before +they are even passed to the individual block drivers, so most if not all +of these are going to be false positives. + +> +> m->nb_clusters here is limited by s->l2_slice_size (see for example +> +> handle_alloc) so I wouldn't be surprised if this is a false positive. I +> +> couldn't find this particular multiplication in Coverity, but it has +> +> about 250 issues marked as intentional or false positive so there's +> +> probably a lot of overlap with what LGTM found. +> +> +> +> Paolo +> +> +From other projects I know that there is a certain overlap between the +> +results from Coverity Scan an LGTM, but it is good to have both +> +analyzers, and the results from LGTM are typically quite reliable. +> +> +Even if we know that there is no multiplication overflow, the code could +> +be modified. Either the assigned value should use the same data type as +> +the factors (possible when there is never an overflow, avoids a size +> +extension), or the multiplication could use the larger data type by +> +adding a type cast to one of the factors (then an overflow cannot +> +happen, static code analysers and human reviewers have an easier job, +> +but the multiplication costs more time). +But if you look at the code we're talking about, you see that it's +complaining about things where being more explicit would make things +less readable. + +For example, if complains about the multiplication in this line: + + s->file_size += n * s->header.cluster_size; + +We know that n * s->header.cluster_size fits in 32 bits, but +s->file_size is 64 bits (and has to be 64 bits). Do you really think we +should introduce another uint32_t variable to store the intermediate +result? And if we cast n to uint64_t, not only might the multiplication +cost more time, but also human readers would wonder why the result could +become larger than 32 bits. So a cast would be misleading. + + +It also complains about this line: + + ret = bdrv_truncate(bs->file, (3 + l1_clusters) * s->cluster_size, + PREALLOC_MODE_OFF, &local_err); + +Here, we don't even assign the result to a 64 bit variable, but just +pass it to a function which takes a 64 bit parameter. Again, I don't +think introducing additional variables for the intermediate result or +adding casts would be an improvement of the situation. + + +So I don't think this is a good enough tool to base our code on what it +does and doesn't understand. It would have too much of a negative impact +on our code. We'd rather need a way to mark false positives as such and +move on without changing the code in such cases. + +Kevin + +On Sat, 13 Jul 2019 at 18:46, Stefan Weil <address@hidden> wrote: +> +LGTM reports 16 errors, 81 warnings and 119 recommendations: +> +https://lgtm.com/projects/g/qemu/qemu/alerts/?mode=list +. +I had a look at some of these before, but mostly I came +to the conclusion that it wasn't worth trying to put the +effort into keeping up with the site because they didn't +seem to provide any useful way to mark things as false +positives. Coverity has its flaws but at least you can do +that kind of thing in its UI (it runs at about a 33% fp +rate, I think.) "Analyzer thinks this multiply can overflow +but in fact it's not possible" is quite a common false +positive cause... + +Anyway, if you want to fish out specific issues, analyse +whether they're false positive or real, and report them +to the mailing list as followups to the patches which +introduced the issue, that's probably the best way for +us to make use of this analyzer. (That is essentially +what I do for coverity.) + +thanks +-- PMM + +Am 14.07.2019 um 19:30 schrieb Peter Maydell: +[...] +> +"Analyzer thinks this multiply can overflow +> +but in fact it's not possible" is quite a common false +> +positive cause... +The analysers don't complain because a multiply can overflow. + +They complain because the code indicates that a larger result is +expected, for example uint64_t = uint32_t * uint32_t. They would not +complain for the same multiplication if it were assigned to a uint32_t. + +So there is a simple solution to write the code in a way which avoids +false positives... + +Stefan + +Stefan Weil <address@hidden> writes: + +> +Am 14.07.2019 um 19:30 schrieb Peter Maydell: +> +[...] +> +> "Analyzer thinks this multiply can overflow +> +> but in fact it's not possible" is quite a common false +> +> positive cause... +> +> +> +The analysers don't complain because a multiply can overflow. +> +> +They complain because the code indicates that a larger result is +> +expected, for example uint64_t = uint32_t * uint32_t. They would not +> +complain for the same multiplication if it were assigned to a uint32_t. +I agree this is an anti-pattern. + +> +So there is a simple solution to write the code in a way which avoids +> +false positives... +You wrote elsewhere in this thread: + + Either the assigned value should use the same data type as the + factors (possible when there is never an overflow, avoids a size + extension), or the multiplication could use the larger data type by + adding a type cast to one of the factors (then an overflow cannot + happen, static code analysers and human reviewers have an easier + job, but the multiplication costs more time). + +Makes sense to me. + +On 7/14/19 5:30 PM, Peter Maydell wrote: +> +I had a look at some of these before, but mostly I came +> +to the conclusion that it wasn't worth trying to put the +> +effort into keeping up with the site because they didn't +> +seem to provide any useful way to mark things as false +> +positives. Coverity has its flaws but at least you can do +> +that kind of thing in its UI (it runs at about a 33% fp +> +rate, I think.) +Yes, LGTM wants you to modify the source code with + + /* lgtm [cpp/some-warning-code] */ + +and on the same line as the reported problem. Which is mildly annoying in that +you're definitely committing to LGTM in the long term. Also for any +non-trivial bit of code, it will almost certainly run over 80 columns. + + +r~ + |