I got the strace output of my non-C binary (I filtered the noise out of the output for you):

mmap(NULL, 8192, PROT_READ | PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)

I also have readelf -l output:

Elf file type is EXEC (Executable file)

Entry point 0x401311

There are 7 program headers, starting at offset 64

Program Headers:

Type Offset VirtAddr PhysAddr

FileSiz MemSiz Flags Align

LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000

0x00000000000db604 0x00000000000db604 R E 1000

LOAD 0x00000000000dc1c0 0x00000000004dd1c0 0x00000000004dd1c0

0x0000000000006220 0x00000000000091dc RW 1000

NOTE 0x00000000000001c8 0x00000000004001c8 0x00000000004001c8

0x0000000000000024 0x0000000000000024 R 4

GNU_EH_FRAME 0x00000000000d5680 0x00000000004d5680 0x00000000004d5680

0x0000000000005f84 0x0000000000005f84 R 4

GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000

0x0000000000000000 0x0000000000000000 RWE 0

TLS 0x00000000000dc1c0 0x00000000004dd1c0 0x00000000004dd1c0

0x0000000000000100 0x0000000000000100 R 10

GNU_RELRO 0x00000000000dc1c0 0x00000000004dd1c0 0x00000000004dd1c0

0x0000000000005e40 0x0000000000005e40 RW 20

Section to Segment mapping:

Segment Sections...

00 .note.gnu.build-id .init .text .fini .gcc_except_table .rodata .debug_gdb_scripts .eh_frame .eh_frame_hdr

01 .tdata .data.rel.ro.local .data.rel.ro .init_array .got .got.plt .data .bss

02 .note.gnu.build-id

03 .eh_frame_hdr

05 .tdata

06 .tdata .data.rel.ro.local .data.rel.ro .init_array .got .got.plt

Some notes:

As a test, I changed the non-C binary's target device file to /dev/zero, and then I could see that the non-C mmap attempt would succeed just fine.

After further verification and debugging based on guidance from another forum, I have convinced that the vm_flags change must be occuring somewhere in kernel land after control flow has left user land. Now I need to figure out how to use a kernel debugger or kprobes to walk through the execution of mmap callback delegation and see where the flags parameter is being changed.

I was pointed out to this:
http://lxr.free-electrons.com/source/mm/mmap.c#L1312

But why would my vm_flags be changed by the kernel? And what can I do to get this to stop? Why is the kernel changing the vm_flags for a non-C binary using my device file, but not for either a C binary using my device file or any type of binary that's not using my device file?

On Thu, Jan 14, 2016 at 12:28 PM, Kenneth Adam Miller <kennethadammiller@gmail.com> wrote:

On Thu, Jan 14, 2016 at 12:00 PM, Mike Krinkin <krinkin.m.u@gmail.com> wrote:
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?

Yes, the former works in so much as mmap completes successfully. I've verified that the
parameters are identical in the non-C program. The issue of just using the C only program
is that the actual implementation of interest is in the non-C program, and that's because
that language facilitates other features that are *required* on our end.

> 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?

By manually, I mean with a print call just before the mmap call in each of the
programs. Right now, I'm working on getting a strace output, but I have to run that in qemu.
To be able to run it in qemu in order to isolate the driver and all from my host, I have to build
with buildroot. So I'll email that when I get it, but it'll be a while.

>
> 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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