[Xen-devel] alloca() in linux_privcmd_map_foreign_bulk causing segfault

[AP <apxeng@xxxxxxxxx>]

On xen-unstable 25164:5bbda657a016, when I try to map in large amounts
of pages (in the GB range) from a guest in to Dom0 using
xc_map_foreign_bulk() I am hitting a segfault.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7bd38d5 in linux_privcmd_map_foreign_bulk (xch=0x605050,
    h=<optimized out>, dom=2, prot=<optimized out>, arr=0x7ffff6bf5010,
    err=0x7ffff67f4010, num=<optimized out>)
    at /usr/include/x86_64-linux-gnu/bits/string3.h:52
52        return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
(gdb) bt
#0  0x00007ffff7bd38d5 in linux_privcmd_map_foreign_bulk (xch=0x605050,
    h=<optimized out>, dom=2, prot=<optimized out>, arr=0x7ffff6bf5010,
    err=0x7ffff67f4010, num=<optimized out>)
    at /usr/include/x86_64-linux-gnu/bits/string3.h:52
#1  0x00007ffff7bd1ffc in xc_map_foreign_bulk (xch=<optimized out>,
    dom=<optimized out>, prot=<optimized out>, arr=<optimized out>,
    err=<optimized out>, num=<optimized out>) at xc_foreign_memory.c:79

This was working for me with Xen 4.1.2. On comparing
linux_privcmd_map_foreign_bulk() between 4.1.2 and unstable I see that
the pfn array in linux_privcmd_map_foreign_bulk() is being allocated
using alloca() in unstable vs malloc() in 4.1.2. So I am blowing the
stack with the call. If I replace the alloca() with malloc() the call
goes through. What is the way around this? Should I be using
xc_map_foreign_batch() instead, which I think is deprecated? Please
advice...

Thanks,
AP

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