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 04 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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