[pull] master from php:master #690
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Debugging memory corruption issues in production can be difficult when it's not possible to use a debug build or ASAN/MSAN/Valgrind (e.g. for performance reasons).
This change makes it possible to enable some basic heap debugging helpers without rebuilding PHP. This is controlled by the environment variable ZEND_MM_DEBUG. The env var takes a comma-separated list of parameters:
- poison_free=byte: Override freed blocks with the specified byte value (represented as a number)
- poison_alloc=byte: Override newly allocated blocks with the specified byte value (represented as a number)
- padding=bytes: Pad allocated blocks with the specified amount of bytes (if non-zero, a value >= 16 is recommended to not break alignments)
- check_freelists_on_shutdown=0|1: Enable checking freelist consistency [1] on shutdown
Example:
ZEND_MM_DEBUG=poison_free=0xbe,poison_alloc=0xeb,padding=16,check_freelists_on_shutdown=1 php ...
This is implemented by installing custom handlers when ZEND_MM_DEBUG is set.
This has zero overhead when ZEND_MM_DEBUG is not set. When ZEND_MM_DEBUG is set, the overhead is about 8.5% on the Symfony Demo benchmark.
Goals:
- Crash earlier after a memory corruption, to extract a useful backtrace
- Be usable in production with reasonable overhead
- Having zero overhead when not enabled
Non-goals:
- Replace debug builds, valgrind, ASAN, MSAN or other sanitizers
[1] #14054
Co-authored-by: Tim Düsterhus <[email protected]>
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