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malloc: Do not use MAP_NORESERVE to allocate heap segments

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Address space for heap segments is reserved in a mmap call with
MAP_ANONYMOUS | MAP_PRIVATE and protection flags PROT_NONE.  This
reservation does not count against the RSS limit of the process or
system.  Backing memory is allocated using mprotect in alloc_new_heap
and grow_heap, and at this point, the allocator expects the kernel
to provide memory (subject to memory overcommit).

The SIGSEGV that might generate due to MAP_NORESERVE (according to
the mmap manual page) does not seem to occur in practice, it's always
SIGKILL from the OOM killer.  Even if there is a way that SIGSEGV
could be generated, it is confusing to applications that this only
happens for secondary heaps, not for large mmap-based allocations,
and not for the main arena.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
This commit is contained in:
Florian Weimer 2022-08-15 16:45:40 +02:00
parent f82e05ebb2
commit 85860ad6ea
2 changed files with 1 additions and 8 deletions
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5
malloc/arena.c
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@ -559,16 +559,13 @@ new_heap (size_t size, size_t top_pad)
#if HAVE_TUNABLES
if (__glibc_unlikely (mp_.hp_pagesize != 0))
{
/* MAP_NORESERVE is not used for huge pages because some kernel may
not reserve the mmap region and a subsequent access may trigger
a SIGBUS if there is no free pages in the pool. */
heap_info *h = alloc_new_heap (size, top_pad, mp_.hp_pagesize,
mp_.hp_flags);
if (h != NULL)
return h;
}
#endif
return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), MAP_NORESERVE);
return alloc_new_heap (size, top_pad, GLRO (dl_pagesize), 0);
}
/* Grow a heap. size is automatically rounded up to a

4
malloc/malloc.c
View File

@ -1110,10 +1110,6 @@ static mchunkptr mremap_chunk(mchunkptr p, size_t new_size);
# define MAP_ANONYMOUS MAP_ANON
#endif
#ifndef MAP_NORESERVE
# define MAP_NORESERVE 0
#endif
#define MMAP(addr, size, prot, flags) \
__mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS|MAP_PRIVATE, -1, 0)