Replace constant stack canary with per-task random

[jserv]

This commit replaces fixed STACK_CANARY (0x33333333U) with per-task random canary generated using xorshift32 PRNG. This prevents trivial canary forgery attacks and significantly improves stack overflow detection.

The xorshift32 PRNG provides sufficient entropy for embedded systems without MMU/MPU support. Future enhancements can improve PRNG seeding with hardware entropy sources.

Close #8

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Summary by cubic

Replaced the fixed stack canary with a per-task random canary to harden stack overflow detection and prevent trivial forgery. Each task gets its own canary from a lightweight xorshift32 PRNG seeded at boot with hardware entropy.

• New Features
  • Added tcb.canary; wrote it to both ends of each task’s stack.
  • Generated canary via random() with hardware-entropy seeding; fallback to 0xDEADBEEF if zero.
  • Updated stack check to validate against the task’s canary and print expected value.

^{Written for commit 4a8087c. Summary will update automatically on new commits.}

[github-actions]

Linmo CI Test Results

Overall Status: ✅ passed
Timestamp: 2025-11-20T04:33:51+00:00

Toolchain Results

Toolchain	Build	Crash Test	Functional
GNU	✅ passed	✅ passed	✅ passed
LLVM	✅ passed	⏭️ skipped	⏭️ skipped

Application Tests

App	GNU	LLVM
cond	✅ passed	⏭️ skipped
coop	✅ passed	⏭️ skipped
cpubench	✅ passed	⏭️ skipped
echo	✅ passed	⏭️ skipped
hello	✅ passed	⏭️ skipped
mqueues	✅ passed	⏭️ skipped
mutex	✅ passed	⏭️ skipped
pipes	✅ passed	⏭️ skipped
pipes_small	✅ passed	⏭️ skipped
pipes_struct	✅ passed	⏭️ skipped
prodcons	✅ passed	⏭️ skipped
progress	✅ passed	⏭️ skipped
rtsched	✅ passed	⏭️ skipped
semaphore	✅ passed	⏭️ skipped
suspend	✅ passed	⏭️ skipped
test64	✅ passed	⏭️ skipped
test_libc	✅ passed	⏭️ skipped
timer	✅ passed	⏭️ skipped
timer_kill	✅ passed	⏭️ skipped

Functional Test Details

Test	GNU	LLVM
mutex:data_consistency	✅ passed	⏭️ skipped
mutex:fairness	✅ passed	⏭️ skipped
mutex:mutual_exclusion	✅ passed	⏭️ skipped
mutex:overall	✅ passed	⏭️ skipped
semaphore:overall	✅ passed	⏭️ skipped

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Report generated from test-summary.toml