From af756d78ac042a2eed2417c5250d4b675d43bf93 Mon Sep 17 00:00:00 2001 From: Adrian Kummerlaender Date: Wed, 17 Feb 2016 15:02:48 +0100 Subject: Implement static allocator for initialization The previous interposition logic based on plain usage of `dlsym` analogously to various online examples led to a deadlock during _neovim_ startup. This deadlock was caused by _neovim_'s custom memory allocation library _jemalloc_ because it calls `mmap` during its initialization phase. The problem with calling `mmap` during initialization is that this already leads to executing `libChangeLog`'s `mmap` version whoes static `actual_mmap` function pointer is not initialized at this point in time. This is detected and leads to a call to `dlsym` to remedy this situation. Sadly `dlsym` in turn requires memory allocation using `calloc` which leads us back to initializing _jemalloc_ and as such to a deadlock. I first saw this as a bug in _jemalloc_ which seemed to be confirmed by a short search in my search engine of choice. This prompted me to create an appropriate [bug report](https://github.com/jemalloc/jemalloc/issues/329) which was dismissed as a problem in the way `mmap` was interposed and not as a bug in the library. Thus it seems to be accepted practice that it is not the responsibility of a custom memory allocator to cater to the initialization needs of other libraries relying on function interposition. This is of course a valid position as the whole issue is a kind of _chicken and egg_ problem where both sides can be argued. To cut to the chase I was left with the only option of working around this deadlock by adapting `libChangeLog` to call `dlsym` without relying on the wrapped application's memory allocator of choice. The most straight forward way to do this is to provide another custom memory allocator alongside the _payload_ function interpositions of `mmap` and friends. `init/alloc.cc` implements such a selectively transparent memory allocator that offers a small static buffer for usage in the context of executing `dlsym`.The choice between forwarding memory allocation requests to the wrapped application's allocator and using the static buffer is governed by `init::dlsymContext`. This tiny helper class maintains an `dlsym_level` counter by posing as a scope guard. The end result of this extension to `libChangeLog` is that it now also works with applications using _jemalloc_ such as _neovim_ and should overall be much more robust during its initialization phase. --- src/init/alloc.cc | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ src/init/alloc.h | 22 ++++++++++++++++++++++ 2 files changed, 72 insertions(+) create mode 100644 src/init/alloc.cc create mode 100644 src/init/alloc.h (limited to 'src/init') diff --git a/src/init/alloc.cc b/src/init/alloc.cc new file mode 100644 index 0000000..d18917b --- /dev/null +++ b/src/init/alloc.cc @@ -0,0 +1,50 @@ +#include "alloc.h" + +#include +#include + +namespace init { + +std::uint8_t dlsym_level = 0; +size_t buffer_offset = 0; +char buffer[4096]; + +void* static_malloc(size_t size) { + if ( buffer_offset + size >= sizeof(buffer) ) { + std::fprintf(stderr, "static allocator out of memory: %lu\n", buffer_offset); + exit(1); + } + + buffer_offset += size; + + return buffer + buffer_offset; +} + +void* static_calloc(size_t block, size_t size) { + void* const mem = static_malloc(block * size); + + for ( size_t i = 0; i < (block * size); ++i ) { + *(static_cast(mem) + i) = '\0'; + } + + return mem; +} + +bool from_static_buffer(void* ptr) { + return static_cast(ptr) >= buffer + && static_cast(ptr) <= buffer + sizeof(buffer); +} + +dlsymContext::dlsymContext() { + ++dlsym_level; +} + +dlsymContext::~dlsymContext() { + --dlsym_level; +} + +bool dlsymContext::is_active() { + return dlsym_level > 0; +} + +} diff --git a/src/init/alloc.h b/src/init/alloc.h new file mode 100644 index 0000000..cd1228d --- /dev/null +++ b/src/init/alloc.h @@ -0,0 +1,22 @@ +#ifndef CHANGE_SRC_INIT_ALLOC_H_ +#define CHANGE_SRC_INIT_ALLOC_H_ + +#include + +namespace init { + +void* static_malloc(size_t size); +void* static_calloc(size_t block, size_t size); + +bool from_static_buffer(void* ptr); + +struct dlsymContext { + dlsymContext(); + ~dlsymContext(); + + static bool is_active(); +}; + +} + +#endif // CHANGE_SRC_INIT_ALLOC_H_ -- cgit v1.2.3