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* mitigate blow-up of heap size under malloc/free contentionRich Felker2015-08-071-14/+14
| | | | | | | | | | | | | | | | | | | | | | | during calls to free, any free chunks adjacent to the chunk being freed are momentarily held in allocated state for the purpose of merging, possibly leaving little or no available free memory for other threads to allocate. under this condition, other threads will attempt to expand the heap rather than waiting to use memory that will soon be available. the race window where this happens is normally very small, but became huge when free chooses to use madvise to release unused physical memory, causing unbounded heap size growth. this patch drastically shrinks the race window for unwanted heap expansion by performing madvise with the bin lock held and marking the bin non-empty in the binmask before making the expensive madvise syscall. testing by Timo Teräs has shown this approach to be a suitable mitigation. more invasive changes to the synchronization between malloc and free would be needed to completely eliminate the problem. it's not clear whether such changes would improve or worsen typical-case performance, or whether this would be a worthwhile direction to take malloc development.
* fix calloc when __simple_malloc implementation is usedRich Felker2015-06-221-0/+11
| | | | | | | | | | | | | | | | | previously, calloc's implementation encoded assumptions about the implementation of malloc, accessing a size_t word just prior to the allocated memory to determine if it was obtained by mmap to optimize out the zero-filling. when __simple_malloc is used (static linking a program with no realloc/free), it doesn't matter if the result of this check is wrong, since all allocations are zero-initialized anyway. but the access could be invalid if it crosses a page boundary or if the pointer is not sufficiently aligned, which can happen for very small allocations. this patch fixes the issue by moving the zero-fill logic into malloc.c with the full malloc, as a new function named __malloc0, which is provided by a weak alias to __simple_malloc (which always gives zero-filled memory) when the full malloc is not in use.
* refactor malloc's expand_heap to share with __simple_mallocRich Felker2015-06-141-58/+28
| | | | | | | | | | | | | | | | | | | this extends the brk/stack collision protection added to full malloc in commit 276904c2f6bde3a31a24ebfa201482601d18b4f9 to also protect the __simple_malloc function used in static-linked programs that don't reference the free function. it also extends support for using mmap when brk fails, which full malloc got in commit 5446303328adf4b4e36d9fba21848e6feb55fab4, to __simple_malloc. since __simple_malloc may expand the heap by arbitrarily large increments, the stack collision detection is enhanced to detect interval overlap rather than just proximity of a single address to the stack. code size is increased a bit, but this is partly offset by the sharing of code between the two malloc implementations, which due to linking semantics, both get linked in a program that needs the full malloc with realloc/free support.
* in malloc, refuse to use brk if it grows into stackRich Felker2015-06-091-1/+9
| | | | | | | | | | | | | | | | | | | | | | | | the linux/nommu fdpic ELF loader sets up the brk range to overlap entirely with the main thread's stack (but growing from opposite ends), so that the resulting failure mode for malloc is not to return a null pointer but to start returning pointers to memory that overlaps with the caller's stack. needless to say this extremely dangerous and makes brk unusable. since it's non-trivial to detect execution environments that might be affected by this kernel bug, and since the severity of the bug makes any sort of detection that might yield false-negatives unsafe, we instead check the proximity of the brk to the stack pointer each time the brk is to be expanded. both the main thread's stack (where the real known risk lies) and the calling thread's stack are checked. an arbitrary gap distance of 8 MB is imposed, chosen to be larger than linux default main-thread stack reservation sizes and larger than any reasonable stack configuration on nommu. the effeciveness of this patch relies on an assumption that the amount by which the brk is being grown is smaller than the gap limit, which is always true for malloc's use of brk. reliance on this assumption is why the check is being done in malloc-specific code and not in __brk.
* remove useless check of bin match in mallocRich Felker2015-03-041-1/+1
| | | | | | | | | | | | | | | | this re-check idiom seems to have been copied from the alloc_fwd and alloc_rev functions, which guess a bin based on non-synchronized memory access to adjacent chunk headers then need to confirm, after locking the bin, that the chunk is actually in the bin they locked. the check being removed, however, was being performed on a chunk obtained from the already-locked bin. there is no race to account for here; the check could only fail in the event of corrupt free lists, and even then it would not catch them but simply continue running. since the bin_index function is mildly expensive, it seems preferable to remove the check rather than trying to convert it into a useful consistency check. casual testing shows a 1-5% reduction in run time.
* fix init race that could lead to deadlock in malloc init codeRich Felker2015-03-041-39/+14
| | | | | | | | | | | | | the malloc init code provided its own version of pthread_once type logic, including the exact same bug that was fixed in pthread_once in commit 0d0c2f40344640a2a6942dda156509593f51db5d. since this code is called adjacent to expand_heap, which takes a lock, there is no reason to have pthread_once-type initialization. simply moving the init code into the interval where expand_heap already holds its lock on the brk achieves the same result with much less synchronization logic, and allows the buggy code to be eliminated rather than just fixed.
* make all objects used with atomic operations volatileRich Felker2015-03-031-5/+5
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | the memory model we use internally for atomics permits plain loads of values which may be subject to concurrent modification without requiring that a special load function be used. since a compiler is free to make transformations that alter the number of loads or the way in which loads are performed, the compiler is theoretically free to break this usage. the most obvious concern is with atomic cas constructs: something of the form tmp=*p;a_cas(p,tmp,f(tmp)); could be transformed to a_cas(p,*p,f(*p)); where the latter is intended to show multiple loads of *p whose resulting values might fail to be equal; this would break the atomicity of the whole operation. but even more fundamental breakage is possible. with the changes being made now, objects that may be modified by atomics are modeled as volatile, and the atomic operations performed on them by other threads are modeled as asynchronous stores by hardware which happens to be acting on the request of another thread. such modeling of course does not itself address memory synchronization between cores/cpus, but that aspect was already handled. this all seems less than ideal, but it's the best we can do without mandating a C11 compiler and using the C11 model for atomics. in the case of pthread_once_t, the ABI type of the underlying object is not volatile-qualified. so we are assuming that accessing the object through a volatile-qualified lvalue via casts yields volatile access semantics. the language of the C standard is somewhat unclear on this matter, but this is an assumption the linux kernel also makes, and seems to be the correct interpretation of the standard.
* avoid malloc failure for small requests when brk can't be extendedRich Felker2014-04-021-1/+23
| | | | | | | | | | | | | | | | | this issue mainly affects PIE binaries and execution of programs via direct invocation of the dynamic linker binary: depending on kernel behavior, in these cases the initial brk may be placed at at location where it cannot be extended, due to conflicting adjacent maps. when brk fails, mmap is used instead to expand the heap. in order to avoid expensive bookkeeping for managing fragmentation by merging these new heap regions, the minimum size for new heap regions increases exponentially in the number of regions. this limits the number of regions, and thereby the number of fixed fragmentation points, to a quantity which is logarithmic with respect to the size of virtual address space and thus negligible. the exponential growth is tuned so as to avoid expanding the heap by more than approximately 50% of its current total size.
* fix failure of malloc to set errno on heap (brk) exhaustionRich Felker2013-10-051-0/+1
| | | | | I wrongly assumed the brk syscall would set errno, but on failure it returns the old value of the brk rather than an error code.
* fix potential deadlock bug in libc-internal locking logicRich Felker2013-09-201-8/+7
| | | | | | | | | | | | | | | | | | | | if a multithreaded program became non-multithreaded (i.e. all other threads exited) while one thread held an internal lock, the remaining thread would fail to release the lock. the the program then became multithreaded again at a later time, any further attempts to obtain the lock would deadlock permanently. the underlying cause is that the value of libc.threads_minus_1 at unlock time might not match the value at lock time. one solution would be returning a flag to the caller indicating whether the lock was taken and needs to be unlocked, but there is a simpler solution: using the lock itself as such a flag. note that this flag is not needed anyway for correctness; if the lock is not held, the unlock code is harmless. however, the memory synchronization properties associated with a_store are costly on some archs, so it's best to avoid executing the unlock code when it is unnecessary.
* harden realloc/free to detect simple overflowsRich Felker2013-07-191-0/+6
| | | | | | | | | | | the sizes in the header and footer for a chunk should always match. if they don't, the program has definitely invoked undefined behavior, and the most likely cause is a simple overflow, either of a buffer in the block being freed or the one just below it. crashing here should not only improve security of buggy programs, but also aid in debugging, since the crash happens in a context where you have a pointer to the likely-overflowed buffer.
* page-align initial brk value used by malloc in shared libcRich Felker2012-12-071-1/+5
| | | | | | | | | | this change fixes an obscure issue with some nonstandard kernels, where the initial brk syscall returns a pointer just past the end of bss rather than the beginning of a new page. in that case, the dynamic linker has already reclaimed the space between the end of bss and the page end for use by malloc, and memory corruption (allocating the same memory twice) will occur when malloc again claims it on the first call to brk.
* workaround gcc got-register-reload performance problems in mallocRich Felker2012-09-141-4/+8
| | | | | | | with this patch, the malloc in libc.so built with -Os is nearly the same speed as the one built with -O3. thus it solves the performance regression that resulted from removing the forced -O3 when building libc.so; now libc.so can be both small and fast.
* fix issue with excessive mremap syscalls on reallocRich Felker2011-11-161-4/+2
| | | | | | | | | | CHUNK_SIZE macro was defined incorrectly and shaving off at least one significant bit in the size of mmapped chunks, resulting in the test for oldlen==newlen always failing and incurring a syscall. fortunately i don't think this issue caused any other observable behavior; the definition worked correctly for all non-mmapped chunks where its correctness matters more, since their lengths are always multiples of the alignment.
* use new a_crash() asm to optimize double-free handler.Rich Felker2011-08-231-2/+2
| | | | | | | gcc generates extremely bad code (7 byte immediate mov) for the old null pointer write approach. it should be generating something like "xor %eax,%eax ; mov %al,(%eax)". in any case, using a dedicated crashing opcode accomplishes the same thing in one byte.
* simplify and improve double-free checkRich Felker2011-08-151-2/+2
| | | | | | | | | | | | a valid mmapped block will have an even (actually aligned) "extra" field, whereas a freed chunk on the heap will always have an in-use neighbor. this fixes a potential bug if mmap ever allocated memory below the main program/brk (in which case it would be wrongly-detected as a double-free by the old code) and allows the double-free check to work for donated memory outside of the brk area (or, in the future, secondary heap zones if support for their creation is added).
* eliminate OOB array hacks in mallocRich Felker2011-06-261-46/+45
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* malloc: cast size down to int in bin_index functionsRich Felker2011-06-121-2/+2
| | | | | | | | | | | even if size_t was 32-bit already, the fact that the value was unsigned and that gcc is too stupid to figure out it would be positive as a signed quantity (due to the immediately-prior arithmetic and conditionals) results in gcc compiling the integer-to-float conversion as zero extension to 64 bits followed by an "fildll" (64 bit) instruction rather than a simple "fildl" (32 bit) instruction on x86. reportedly fildll is very slow on certain p4-class machines; even if not, the new code is slightly smaller.
* use volatile pointers for intentional-crash code.Rich Felker2011-06-061-2/+2
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* namespace fixes for sys/mman.hRich Felker2011-04-201-0/+1
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* fix rare but nasty under-allocation bug in malloc with large requestsRich Felker2011-04-041-1/+1
| | | | | | | | | the bug appeared only with requests roughly 2*sizeof(size_t) to 4*sizeof(size_t) bytes smaller than a multiple of the page size, and only for requests large enough to be serviced by mmap instead of the normal heap. it was only ever observed on 64-bit machines but presumably could also affect 32-bit (albeit with a smaller window of opportunity).
* avoid over-allocation of brk on first mallocRich Felker2011-04-011-4/+4
| | | | | | | if init_malloc returns positive (successful first init), malloc will retry getting a chunk from the free bins rather than expanding the heap again. also pass init_malloc a hint for the size of the initial allocation.
* very cheap double-free checks in mallocRich Felker2011-03-231-0/+4
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* make malloc(0) return unique pointers rather than NULLRich Felker2011-02-201-5/+9
| | | | | | | this change is made with some reluctance, but i think it's for the best. correct programs must handle either behavior, so there is little advantage to having malloc(0) return NULL. and i managed to actually make the malloc code slightly smaller with this change.
* initial check-in, version 0.5.0 v0.5.0Rich Felker2011-02-121-0/+515