Date: Thu, 11 Aug 2011 11:43:05 +0400 From: Vasiliy Kulikov <segoon@...nwall.com> To: musl@...ts.openwall.com Subject: Re: malloc and linux memory layout On Wed, Aug 10, 2011 at 15:56 -0400, Rich Felker wrote: > Each process has its own 32- or 64-bit virtual address space. > Initially, from bottom to top, it looks something like: > > [low unmappable pages] > [main program text (code) segment] > [main program data segment] > [brk segment (heap)] > [....lots of open virtual address space...] > [mmap zone] > [main thread stack] > [reserved for kernelspace use] > > (Note that there will be small randomized amounts of empty/unused > address space between these regions if ALSR is enabled.)x > > The brk segment starts just above the program's static data and grows > upward into the big open space. Btw, (randomize_va_space): http://www.mjmwired.net/kernel/Documentation/sysctl/kernel.txt > The mmap zone (where mmaps are put by > default) starts just below the stack limit and continues downward as > more mappings are made. Not only this zone. mmap() can return address before main program text if there is enough space there. Other minor (Linux-specific) things: Low unmappable pages region might be absent if the task has CAP_SYS_RAWIO capability. But pages before mmap_min_addr will be mmap'ed only by explicit mmap(addr, ..., MAP_FIXED, ...), no libs will be there. "Reserved for kernelspace use" region might be absent too for 32-bit tasks running on 286-64 system. There could be some specific pages after the stack. On x86-64 it is VDSO and vsyscall pages. -- Vasiliy
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