On 2017-05-15 20:41, Max Reitz wrote: > On 2017-05-12 21:47, John Snow wrote: >> >> >> On 05/12/2017 03:46 PM, Eric Blake wrote: >>> 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. > > We're not really fine because that conversion happens when the result of > qemu_opt_get_size() (a uint64_t) is stored in size (an int64_t). > >>> 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). > > Right. Which is why I said I won't even start on it, but of course I > did. O:-) > >>>> 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. >>> >> >> I had considered the issue separate; but you're welcome to either write >> a patch or squish it into this one, I'm not going to be picky. > > Yep, it is a separate issue, just related. :-) > > But since you and Eric agree, I've squashed it in and thus I'm more than > glad to thank you very much and announce this patch as applied to my > block branch: > > https://github.com/XanClic/qemu/commits/block
...well, so much for that. I'll have to unstage it again because it breaks a bunch of iotests (41 85 96 118 139 141 144 155 156) due to failing to acquire image locks. :-/ I suspect this is because the backing file is opened somewhere and trying to open it breaks now with the locking series applied. Max
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