odd mmap behavior

Carl Banks pavlovevidence at gmail.com
Wed Oct 21 14:02:54 CEST 2009


On Oct 20, 5:03 pm, Brett <brettgra... at gmail.com> wrote:
> I'm trying to read and write from /dev/mem on a linux system using the
> mmap module. When I run this minimal example:
> -------------------------------
> import os, mmap
>
> MAP_MASK = mmap.PAGESIZE - 1
>
> addr = 0x48088024
>
> f = os.open("/dev/mem", os.O_RDWR | os.O_SYNC)
> m = mmap.mmap(f, mmap.PAGESIZE, mmap.MAP_SHARED, mmap.PROT_WRITE |
> mmap.PROT_READ, offset=addr & ~MAP_MASK)
> m.seek(addr & MAP_MASK)
> c = m.read_byte()
> print c
> m.close()
> os.close(f)
> -------------------------------
>
> I get the following error:
> Unhandled fault: external abort on non-linefetch (0x1018) at
> 0x40020024
> Bus error
>
> and python crashes. What is odd is that if I try to read the same
> memory address using the devmem2 program (i'll attach the source at
> the end), everything works fine. Here are the truncated straces for
> each program:
>
> ----- devmem2 0x48088024 -----
> open("/dev/mem", O_RDWR|O_SYNC)         = 3
> fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
> mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
> 0) = 0x4001f000
> write(1, "/dev/mem opened.\n", 17)      = 17
> mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0x48088) =
> 0x40020000
> write(1, "Memory mapped at address 0x40020"..., 37) = 37
> write(1, "Value at address 0x48088024 (0x4"..., 46) = 46
> munmap(0x40020000, 4096)                = 0
> close(3)                                = 0
> io_submit(0, 0, 0xfbad2088 <unfinished ... exit status 0>
> Process 1635 detached
>
> ------ the above minimal python example -----
> open("/dev/mem", O_RDWR|O_SYNC|O_LARGEFILE) = 3
> fstat64(3, {st_mode=S_IFCHR|0640, st_rdev=makedev(1, 1), ...}) = 0
> dup(3)                                  = 4
> mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0x48088) =
> 0x40020000
> --- SIGBUS (Bus error) @ 0 (0) ---
> +++ killed by SIGBUS +++
> Process 1633 detached
>
> Am I using mmap incorrectly?

Looks like you're using it ok to me.

One thing to keep in mind is that the offset parameter was added to
mmap recently, only in Python 2.6 I think, so it might not have been
so battle-tested as the rest of Python.  Also, I would guess the file-
like methods (seek, read, etc.) are not commonly used.  Perhaps those
methods don't play nice with the offset method?  It's possible the
mmap object doesn't recon the seek pointer relative to the offset,
meaning you'd have to use m.seek(addr) instead of m.seek(addr &
MAP_MASK)

Another possibility is that the mmap object is accessing the memory
with bad alignment (when I see bus error the first thing I think of is
that a word was read from a non-word-aligned address).  You don't
happen to be running this on a Power PC?  What happens when you try
accessing the mmap data like an regular array?

All speculation, but my gut feeling is bug in mmap module.


Carl Banks



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