script: Fix buffer over and under flows in script_buftoenv

[rsmarples]

Enforce we have a buffer and it is terminated.
Rework code so that we no longer assert.

Reported by NVIDIA Project Vanessa

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: e350b505-aa55-4660-acd3-83916b40b1a1

📥 Commits

Reviewing files that changed from the base of the PR and between e8a17b9 and 122e908.

📒 Files selected for processing (1)

• src/script.c

🚧 Files skipped from review as they are similar to previous changes (1)

• src/script.c

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Walkthrough

script_buftoenv() in src/script.c is hardened: it now rejects zero-length buffers, validates NUL termination of the input buffer, counts strings by skipping consecutive NUL bytes, and reworks envp pointer array construction to tolerate leading empty strings and consecutive NUL separators. The assert.h include is removed.

Changes

script_buftoenv() Hardening

Layer / File(s)	Summary
Buffer validation and NUL-tolerant string counting src/script.c	Removes assert.h. Adds early return for zero-length buffer. Validates that the last byte is NUL (sets EINVAL and returns NULL if not). Replaces strict string-count loop with one that skips consecutive NUL bytes.
envp pointer array construction rework src/script.c	Replaces the prior pointer-advancement and "avoid last terminator" strategy with a loop that optionally skips leading empty strings when the buffer begins with NUL, then captures one pointer per variable while skipping consecutive NUL separators.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main purpose of the PR: fixing buffer over and underflows in the script_buftoenv function.
Description check	✅ Passed	The description is directly related to the changeset, explaining the enforcement of buffer validation and the elimination of assertions, which aligns with the summary of changes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch script

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/script.c`:
- Around line 167-171: The code in script_buftoenv() checks if len is zero but
does not validate that buf is not NULL before dereferencing it at buf[len - 1].
Add a NULL pointer check for buf before accessing buf[len - 1] to prevent
crashes when NULL is passed with a non-zero length. Additionally, set errno when
returning NULL in the zero-length case to avoid callers encountering stale error
values.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 7913e605-9ad1-4ef0-b7f1-c95c6d84c279

📥 Commits

Reviewing files that changed from the base of the PR and between 7891a22 and e8a17b9.

📒 Files selected for processing (1)

• src/script.c