fix(profiling): Fix I/O profiling gotchas

[realFlowControl]

Description

This PR addresses several correctness bugs, security improvements, and hot-path performance bottlenecks in the I/O profiler (profiling/src/io):

• Correctness in observed_poll: Correctly distinguishes returned and requested events, evaluates both read and write readiness across all descriptors, handles output-only events such as POLLHUP, and avoids reading the descriptor array when nfds == 0 or poll() fails.
• Byte counting in fread and fwrite: Records total bytes (items * size) rather than item count (nobj).
• GNU_RELRO security: Restores PROT_READ memory protection on Linux after patching GOT entries in RELRO segments.
• Loader safety: Adds bounds checks to Mach-O indirect symbol and string tables, masks INDIRECT_SYMBOL_LOCAL/ABS flags, bounds ELF dynamic section loops, and skips the ld-musl linker.
• Fast-path bypass: Non-PHP threads, such as cURL async DNS resolver threads, exit immediately from profiling hooks, avoiding clock reads and sampler TLS initialization. The close hook still invalidates the process-wide file descriptor cache.
• Removed static mut ORIG_*: All hooks now call libc::* directly because ddtrace.so explicitly excludes itself from GOT hooking, eliminating data races and redundant dlsym(RTLD_NEXT) lookups.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

https://datadoghq.atlassian.net/browse/PROF-15825

[datadog-datadog-prod-us1]

 

⚠️ Warnings

❌ Your PR has failed checks. Please review the issues below and take necessary action before merging.

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DataDog/apm-reliability/dd-trace-php | appsec integration tests (ssi): [test8.3-release-ssi]

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ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 54.99% (+0.00%)

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[pr-commenter]

Benchmarks [ profiler ]

Benchmark execution time: 2026-09-04 14:54:05

Comparing candidate commit 8d724d8 in PR branch florian/io-profiling-fixes with baseline commit 3b1267d in branch master.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 24 metrics, 10 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

scenario:php-profiler-timeline-memory-control

• 🟥 cpu_user_time [+32.333ms; +44.230ms] or [+4.976%; +6.807%]
• 🟥 execution_time [+34.292ms; +36.744ms] or [+4.897%; +5.247%]