[Qemu-devel] problems with mmap on qemu-arm

[Qemu-devel] problems with mmap on qemu-arm

[Kai Backman]

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I'm running qemu-arm and see the following output from mmap with 'strace
qemu-arm -strace' (for both host and traget strace output):
mmap(NULL, 32800, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x2ad329b46000
4023 mmap(0,131072,7,34,-1,0) = 0xfffffff2

When my program later tries to read memory at 0xfffffffe it fails with a
SIGSEGV:
Program received signal SIGSEGV, Segmentation fault.
1: x/i $pc  0xce30:     ldr     r1, [r5]
(gdb) i r
r5             0xfffffffe       -2

Why does qemu-arm return such high addresses on a 32 bit platform and why is
the memory inaccessible? If this doesn't seem like an obvious bug in my
code, where in the qemu code should I start looking for how mmap works?

 Kai

--
Kai Backman, Software Engineer, kaib@google.com

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[Qemu-devel] Re: problems with mmap on qemu-arm

[Kai Backman]

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I tracked this down to the code in linux-user/syscall.c. The mmap code seems
broken, it assumes arg1 is a pointer to an array where the real arguments
are. Given arg1 = 0 the memory access check fails.
The code I'm working on will happily take mmap2 instead so I have a
workaround. Does someone have an opinion on the state of the mmap code?

 Kai

On Wed, Jun 17, 2009 at 8:54 PM, Kai Backman <kaib@google.com> wrote:

> I'm running qemu-arm and see the following output from mmap with 'strace
> qemu-arm -strace' (for both host and traget strace output):
> mmap(NULL, 32800, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> 0x2ad329b46000
> 4023 mmap(0,131072,7,34,-1,0) = 0xfffffff2
>
> When my program later tries to read memory at 0xfffffffe it fails with a
> SIGSEGV:
> Program received signal SIGSEGV, Segmentation fault.
> 1: x/i $pc  0xce30:     ldr     r1, [r5]
> (gdb) i r
> r5             0xfffffffe       -2
>
> Why does qemu-arm return such high addresses on a 32 bit platform and why
> is the memory inaccessible? If this doesn't seem like an obvious bug in my
> code, where in the qemu code should I start looking for how mmap works?
>
>  Kai
>
> --
> Kai Backman, Software Engineer, kaib@google.com
>



-- 
Kai Backman, Software Engineer, kaib@google.com

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Re: [Qemu-devel] Re: problems with mmap on qemu-arm

[Riku Voipio]

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On Wed, Jun 17, 2009 at 11:38:32PM +0300, Kai Backman wrote:
>    I tracked this down to the code in linux-user/syscall.c. The mmap code
>    seems broken, it assumes arg1 is a pointer to an array where the real
>    arguments are. Given arg1 = 0 the memory access check fails.
>    The code I'm working on will happily take mmap2 instead so I have a
>    workaround. Does someone have an opinion on the state of the mmap code?
>     Kai

looking at old_mmap() for arm in kernel, qemu seems to the right thing. Ie
you are probably calling mmap the wrong way, you are supposed to give it a
array.

mmap syscall is depreceated, glibc uses mmap2 since forever.

>    On Wed, Jun 17, 2009 at 8:54 PM, Kai Backman <[1]kaib@google.com> wrote:
> 
>      I'm running qemu-arm and see the following output from mmap with 'strace
>      qemu-arm -strace' (for both host and traget strace output):
>      mmap(NULL, 32800, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
>      0) = 0x2ad329b46000
>      4023 mmap(0,131072,7,34,-1,0) = 0xfffffff2
>      When my program later tries to read memory at 0xfffffffe it fails with a
>      SIGSEGV:
>      Program received signal SIGSEGV, Segmentation fault.
>      1: x/i $pc  0xce30:     ldr     r1, [r5]
>      (gdb) i r
>      r5             0xfffffffe       -2
>      Why does qemu-arm return such high addresses on a 32 bit platform and
>      why is the memory inaccessible? If this doesn't seem like an obvious bug
>      in my code, where in the qemu code should I start looking for how mmap
>      works?
>       Kai
>      --
>      Kai Backman, Software Engineer, [2]kaib@google.com
> 
>    --
>    Kai Backman, Software Engineer, [3]kaib@google.com
> 
> References
> 
>    Visible links
>    1. mailto:kaib@google.com
>    2. mailto:kaib@google.com
>    3. mailto:kaib@google.com