On 05/12/2017 01:07 PM, Max Reitz wrote: > On 2017-05-11 20:27, John Snow wrote: >> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1213786 >> >> Or, rather, force the open of a backing image if one was specified >> for creation. Using a similar -unsafe option as rebase, allow qemu-img >> to ignore the backing file validation if possible. >>
>> +++ b/block.c >> @@ -4275,37 +4275,37 @@ void bdrv_img_create(const char *filename, const >> char *fmt, >> // The size for the image must always be specified, with one exception: >> // If we are using a backing file, we can obtain the size from there >> size = qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 0); >> - if (size == -1) { > > "Hang on, why should this be -1 when the defval is 0? Where does the -1 > come from?" > "..." > "Oh, the option exists and is set to -1? Why is that?" > "..." > "Oh, because this function always sets it itself, and because @img_size > is set to (uint64_t)-1." I had pretty much the same conversation on my v1 review. https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg01097.html > > First, I won't start with how signed integer overflow is > implementation-defined in C because I hope you have thrashed that out > with Eric (I hope that "to thrash out" is a good translation for > "auskaspern" (lit. "to buffoon out").). Sounds like a reasonable choice of words, even if I don't speak the counterpart language to validate your translation. (uint64_t)-1 is well-defined in C (so I think we're just fine here). But (int64_t)UINT64_MAX is where signed integer overflow does indeed throw wrinkles at you. I seem to recall that qemu has chosen to use compiler flags and/or assumptions that we are using 2s-complement arithmetic with sane behavior (that is, tighter behavior than the bare minimum that C requires), because it was easier than auditing our code for strict C compliance on border cases of conversions from unsigned to signed that trigger undefined behavior. But again, I don't think it affects this patch (where our conversion is only from signed to unsigned, and that is well-defined behavior). > > Second, well, at least we should put -1 as the default value here, then. Indeed, now that two reviewers have tripped on it, qemu_opt_get_size(,,-1) would be nicer. > > Not strictly your fault or something that you need to fix, but it is > just a single line in the vicinity... > > Let me know if you want to address this, for now I'll leave a > > Reviewed-by: Max Reitz <mre...@redhat.com> > > here if you don't want to. I'm okay whether you want to squash that fix into this patch, or whether you do it as a separate followup patch. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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