It appears that the implementation of Curl_verifyhost in x509asn1.c does a memcmp of &addr and q (where q = Curl_getASN1Element(...)). Running with a debugger appears to show that (name.end - q) = 0, which when compared to addrlen fails, preventing the memcmp even happening.
I wonder if it should it be comparing addrlen to (name.end - name.beg) & memcmp-ing &addr with name.beg?
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Yeah that doesn't look right. On success the pointer assigned to q should always be the same as what was assigned to name.end which means it would always be 0. I almost can't believe nobody's ever caught that though it must not be a heavily used function since it's gskit. I agree your solution sounds correct. Are you able to test it out and write up a PR for it?
field of a certificate.
For IP addresses in the subject alternative name field, the length
of the IP address (and hence the number of bytes to perform a
memcmp on) is incorrectly calculated to be zero. The code previously
subtracted q from name.end. where in a successful case q = name.end
and therefore addrlen equalled 0. The change modifies the code to
subtract name.beg from name.end to calculate the length correctly.