qemu on ARM hosts asserts due to code buffer/libc heap conflict

[Peter Maydell]

On ARM hosts qemu (about half the time) asserts on startup:

qemu-system-i386: malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) -
__builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.

This turns out to be because code_gen_alloc() is using mmap(MAP_FIXED) to map the code buffer at address 0x01000000UL, which is in the area glibc happens to be using for its heap. This tends to make the next malloc() abort, although occasionally the stars align and we pass that and fail weirdly later on.

I suspect we need to drop the MAP_FIXED requirement and fix the TCG code to cope with emitting code for longer-range
branches for calls to host fns etc (calls/branches within the generated code should be ok to keep using the short-range
branch insn I think). There is already no guarantee that the generated code and the host C code are within short
branch range of each other...

[Dr. David Alan Gilbert (davidgil-uk)]

Fix pushed into QEMU upstream today.

[Peter Maydell (pmaydell)]

NB: this would typically only happen when the ARM host had address space randomization including brk randomization enabled (ie /proc/sys/kernel/randomize_va_space is 2).