On 2/5/21 4:28 AM, Richard W.M. Jones wrote: > On Thu, Feb 04, 2021 at 01:07:06PM -0600, Eric Blake wrote: >> We have multiple clients of qemu_strtosz (qemu-io, the opts visitor, >> the keyval visitor), and it gets annoying that edge-case testing is >> impacted by implicit rounding to 53 bits of precision due to parsing >> with strtod(). As an example posted by Rich Jones: >> $ nbdkit memory $(( 2**63 - 2**30 )) --run \ >> 'build/qemu-io -f raw "$uri" -c "w -P 3 $(( 2**63 - 2**30 - 512 )) 512" ' >> write failed: Input/output error >> >> because 9223372035781033472 got rounded to 0x7fffffffc0000000 which is >> out of bounds. >> >> It is also worth noting that our existing parser, by virtue of using >> strtod(), accepts decimal AND hex numbers, even though test-cutils >> previously lacked any coverage of the latter. We do have existing >> clients that expect a hex parse to work (for example, iotest 33 using >> qemu-io -c "write -P 0xa 0x200 0x400"), but strtod() parses "08" as 8 >> rather than as an invalid octal number, so we know there are no >> clients that depend on octal. Our use of strtod() also means that >> "0x1.8k" would actually parse as 1536 (the fraction is 8/16), rather >> than 1843 (if the fraction were 8/10); but as this was not covered in >> the testsuite, I have no qualms forbidding hex fractions as invalid, >> so this patch declares that the use of fractions is only supported >> with decimal input, and enhances the testsuite to document that. >> >> Our previous use of strtod() meant that -1 parsed as a negative; now >> that we parse with strtoull(), negative values can wrap around module > > ^^ modulo > > The patch looked fine to me although Vladimir found some problems > which I didn't spot. I have a question: What happens with leading or > trailing whitespace? Is that ignored, rejected or impossible?
leading whitespace: ignored (because both strtod() pre-patch, and now strtoull() post-patch, do so for free). And that is why we have to memchr() (and not strchr(), as pointed out by Vladimir) for a '-' sign, because merely checking *nptr=='-' would be wrong in the presence of leading space. trailing whitespace: treated the same as any other trailing garbage (again, what strtod() and strtoull() give you for free). If endptr was non-NULL, then *endptr now points to that trailing space; if it was NULL, the parse is rejected because of the trailing garbage. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org