mmap(2) segaults with certain len values and MAP_ANON|MAP_FIXED
alexbestms at math.uni-muenster.de
Wed Oct 21 15:30:54 UTC 2009
Robert Watson schrieb am 2009-10-21:
> On Wed, 21 Oct 2009, Alexander Best wrote:
> >this code serves only one purpose: to trigger a segfault. i don't
> >use the code for any other purpose. i was under the impression that
> >mmap() should either succeed or fail (tertium non datur). mmap's
> >manual doesn't say anything about mmap() causing segfaults.
> Have you tried ktracing the application? I think you'll find that
> mmap(2) system call succeeded fine, and that the segfault comes from
> attempting to execute the address in libc on return to userspace, as
> a result of libc not being at that address anymore (since you
> removed its mapping). You can use procstat -v to inspect address
> space use by processes, but as a general rule you don't want to pass
> anything other than an address of 0x0 to mmap(2) unless you're very
> carefully managing the address space of the process. Many userspace
> libraries are involved in using that address space, but especially
> the runtime linker which begins execution in userspace when a binary
> is started.
> Robert N M Watson
> Computer Laboratory
> University of Cambridge
you're right. this kdump shows that the segfault isn't being caused by the
88343 mmap_test CALL
88343 mmap_test RET mmap 4096/0x1000
88343 mmap_test PSIG SIGSEGV SIG_DFL
88343 mmap_test NAMI "mmap_test.core"
thanks for clearing things up.
however i stil think mentioning this situation in the mmap(2) manual (maybe in
section MAP_FIXED) would be a good idea.
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