Inexplicable PROT_EXEC flag set on mmap callback

Mike Krinkin krinkin.m.u at
Thu Jan 14 12:00:12 EST 2016

Hi, i have a couple of questions to clarify, if you don't mind

On Thu, Jan 14, 2016 at 11:04:28AM -0500, Kenneth Adam Miller wrote:
> I have a custom drive and userland program pair that I'm using for a very
> special use case at my workplace where we are mapping specific physical
> address ranges into userland memory with a mmap callback. Everything works
> together well with a C userland program that calls into our driver's ioctl
> and mmap definitions, but for our case we are using an alternative systems
> language just for the userland program.

So you have userland app written in C, and another not written in C?
The former works well while the latter doesn't, am i right?

> That mmap call is failing (properly
> as we want) out from the driver's mmap implementation due to the fact that
> the vm_flags have the VM_EXEC flag set. We do not want users to be able to
> map the memory range as executable, so the driver should check for this as
> it does. The issue is in the fact that somewhere between where mmap is
> called and when the parameters are given to the driver, the vma->vm_flags
> are being set to 255. I've manually checked the values being given to the
> mmap call in our non-C binary, and they are *equivalent* in value to that
> of the C program.

By "manually" do you mean strace? Could you show strace output for
both apps? And also could you show readelf -l output for both binaries?

> My question is, is there anything that can cause the vma->vm_flags to be
> changed in the trip between when the user land program calls mmap and when
> control is delivered to the mmap callback?

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