dnsd: fix bounds check in read_https_alpn_part() - #22420
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"The check `i > 256` permits `i == 256` to pass through. When `i` is then cast to `uint8_t` in `blob_add(b, (uint8_t)i)`, the value wraps to 0, silently encoding a zero-length ALPN entry instead of rejecting it. The condition should be `i > 255` (or equivalently `i >= 256`) to correctly reject any length that does not fit in a single byte." Reported by GitHub Code Quality
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Pull request overview
This PR fixes an off-by-one bounds check in the test DNS server’s HTTPS ALPN parsing (read_https_alpn_part()), ensuring that ALPN entry lengths that do not fit in a single byte are correctly rejected instead of silently wrapping to zero.
Changes:
- Reject
i == 256(and larger) ALPN part lengths by changing the check fromi > 256toi >= 256, preventing(uint8_t)iwraparound to 0.
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vszakats
changed the title
read_https_alpn_part()read_https_alpn_part()
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"The check
i > 256permitsi == 256to pass through. Wheniisthen cast to
uint8_tinblob_add(b, (uint8_t)i), the value wraps to0, silently encoding a zero-length ALPN entry instead of rejecting it.
The condition should be
i > 255(or equivalentlyi >= 256) tocorrectly reject any length that does not fit in a single byte."
Reported by GitHub Code Quality
Follow-up to 86f1e5b #21299