[gdb] Fix gdb crash when reading core file

[gdb] Fix gdb crash when reading core file

Consider the test-case from this patch, compiled with O0.

The executable segfaults, and generates a core dump:
...
$ ./a.out
Segmentation fault (core dumped)
...

When loading the core file, limiting stack size to 4MB, gdb crashes:
...
$ ulimit -s 4096
$ gdb -batch ./a.out core.saved
[New LWP 19379]
Segmentation fault (core dumped)
...

The crash originates here in linux_vsyscall_range_raw, where we call alloca
with phdrs_size == 4194112 (roughly 4MB):
...
      phdrs = (Elf_Internal_Phdr *) alloca (phdrs_size);
...

While for this test-case gdb runs fine with the system default stack limit of
8MB, there are cases reported of 12MB phdrs_size where gdb also crashes with
the system default stack limit.

Fix this by using xmalloc instead of alloca, which prevents the crash provided
the stack limit is at least 112kb.

Build and reg-tested on x86_64-linux.

2018-11-06  Tom de Vries  <tdevries@suse.de>

	* linux-tdep.c (linux_vsyscall_range_raw): Use xmalloc to allocate
	program headers.

	* gdb.base/many-headers.c: New test.
	* gdb.base/many-headers.exp: New file.