GH-49752: [C++][Gandiva] Fix potential buffer overrun in gandiva ssl function

[lriggs]

Rationale for this change

Fixes security related problems found in gdv_hash_using_openssl. Those problems were not deemed to be a security risk.

What changes are included in this PR?

[hash_utils.h:41, hash_utils.cc:66] Removed GANDIVA_EXPORT from gdv_hash_using_openssl — it's an internal helper, not part of the public API.

[hash_utils.cc:105] Changed && → || in the validation condition. The original only errored when both checks failed; now it errors when either result_length != hash_digest_size or result_buf_size != (2 * hash_digest_size).

[hash_utils.cc:135] Fixed snprintf buffer size, so it correctly accounts for the already-written bytes and prevents potential out-of-bounds writes. Allocate result_buf_size + 1 bytes — the extra byte absorbs the final null terminator. Pass result_buf_size - result_buff_index + 1 to snprintf — reflects the actual remaining space (2 hex chars + 1 null = 3 bytes on the last call), preventing any potential overflow if the format ever changed.

Are these changes tested?

Yes, unit tests.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Gandiva] gdv_hash_using_openssl() has some problems #49752

[Copilot]

Pull request overview

This PR addresses correctness/safety issues in Gandiva’s OpenSSL-based hashing helper (gdv_hash_using_openssl) and reduces its intended API exposure.

Changes:

• Stops exporting gdv_hash_using_openssl (intended to be internal).
• Fixes validation logic so mismatched digest size or result buffer size triggers an error.
• Adjusts result-buffer allocation and snprintf sizing to avoid potential out-of-bounds writes when hex-encoding the digest.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cpp/src/gandiva/hash_utils_test.cc	Adds an OpenSSL header include in the hash utils unit tests.
cpp/src/gandiva/hash_utils.h	Removes GANDIVA_EXPORT from gdv_hash_using_openssl declaration.
cpp/src/gandiva/hash_utils.cc	Fixes validation condition and tightens result buffer allocation / snprintf bounds.

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