Messages in this thread | | | Date | Thu, 22 Sep 2016 17:50:30 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH 5/5] arm64: Add uprobe support |
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On Thu, Sep 22, 2016 at 08:53:28AM +0530, Pratyush Anand wrote: > On 21/09/2016:06:04:04 PM, Catalin Marinas wrote: > > On Wed, Sep 21, 2016 at 04:30:47PM +0530, Pratyush Anand wrote: > > > On 20/09/2016:05:59:46 PM, Catalin Marinas wrote: > > > > > +int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, > > > > > + unsigned long addr) > > > > > +{ > > > > > + probe_opcode_t insn; > > > > > + > > > > > + /* TODO: Currently we do not support AARCH32 instruction probing */ > > > > > > > > Is there a way to check (not necessarily in this file) that we don't > > > > probe 32-bit tasks? > > > > > > - Well, I do not have complete idea about it that, how it can be done. I think > > > we can not check that just by looking a single bit in an instruction. > > > My understanding is that, we can only know about it when we are executing the > > > instruction, by reading pstate, but that would not be useful for uprobe > > > instruction analysis. > > > > > > I hope, instruction encoding for aarch32 and aarch64 are different, and by > > > analyzing for all types of aarch32 instructions, we will be able to decide > > > that whether instruction is 32 bit trace-able or not. Accordingly, we can use > > > either BRK or BKPT instruction for breakpoint generation. > > > > We may have some unrelated instruction encoding overlapping but I > > haven't checked. I was more thinking about whether we know which task is > > being probed and check is_compat_task() or maybe using > > compat_user_mode(regs). > > I had thought of this, but problem is that we might not have task in existence > when we enable uprobes. For example: Lets say we are inserting a trace probe at > offset 0x690 in a executable binary. > > echo "p test:0x690" > /sys/kernel/debug/tracing/uprobe_events > echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable > > In the 'enable' step, it is decided that whether instruction is traceable or > not. > > (1) But at this point 'test' executable might not be running. > (2) Even if it is running, is_compat_task() or compat_user_mode() might not be > usable, as they work with 'current' task.
What I find strange is that uprobes allows you to insert a breakpoint instruction that's not even compatible with the task (so it would SIGILL rather than generate a debug exception).
> What I was thinking that, let it go with 'TODO' as of now.
Only that I don't have any guarantee that someone is going to fix it ;).
As a quick workaround you could check mm->task_size > TASK_SIZE_32 in the arch_uprobe_analyze_insn() function.
> Later on, we propose some changes in core layer, so that we can read the elf > headers of executable binary. ELFCLASS will be able to tell us, whether its a 32 > bit or 64 bit executable. I think, moving "struct uprobe" from > kernel/events/uprobes.c to a include/linux header file will do the job. "struct > arch_uprobe" is part of "struct uprobe". "struct arch_uprobe" is passed in > arch_uprobe_analyze_insn(). So, we can access struct uprobe's "inode" element > with this change.
You can get access to struct linux_binfmt via mm_struct but it doesn't currently help much since all the members of this structure point to static functions. Maybe an enum in struct linux_binfmt with format types exposed to the rest of the kernel?
-- Catalin
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