[gnutls-devel] libtasn1 | coding: rewrite _asn1_tag_der() to avoid the tag buffer overrun (#49) (!126)

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Thomas Grainger created a merge request: https://gitlab.com/gnutls/libtasn1/-/merge_requests/126

Project:Branches: graingert/libtasn1:fix-asn1-tag-der-overrun to gnutls/libtasn1:master
Author:   Thomas Grainger



Rewrites `_asn1_tag_der()` as a single pass that writes the tag number straight to the output, most significant digit first, emitting at most `ASN1_MAX_TAG_SIZE - 1` continuation bytes. The result always fits in `ASN1_MAX_TAG_SIZE`, so the one-byte overrun in the old reverse-copy loop (issue #49) is impossible by construction, as suggested by @jas in the issue.

The old truncation behaviour for over-long tags is preserved.

Adds `tests/Test_tag.c` covering:
* positive: `[APPLICATION 200000]` (needs the full three base-128 tag bytes) encodes to the known-good DER;
* negative: `[APPLICATION 2097152]` (a tag too large to encode) stays within `ASN1_MAX_TAG_SIZE` — before the rewrite this produced a nine-byte tag and AddressSanitizer reports a stack-buffer-overflow in `_asn1_tag_der`.

Closes #49

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