Does android support the PTRACE_SINGLESTEP?

Question

OK, this is a simple question.Does android support the PTRACE_SINGLESTEP when I use ptrace systemcall? when I want to ptrace a android apk program, I find that I can\'t pr

Answer 1

PTRACE_SINGLESTEP has been removed on ARM Linux since 2011, by this commit.

The HW has no support for single-stepping; previous kernel support involved decoding the instruction to figure out which one's next (branches) and temporarily replacing it with a debug-break software breakpoint.

Quoting a mailing list message about the same commit, describing the old situation: http://lists.infradead.org/pipermail/linux-arm-kernel/2011-February/041324.html

PTRACE_SINGLESTEP is a ptrace request designed to offer single-stepping support to userspace when the underlying architecture has hardware support for this operation.

On ARM, we set arch_has_single_step() to 1 and attempt to emulate hardware single-stepping by disassembling the current instruction to determine the next pc and placing a software breakpoint on that location.

Unfortunately this has the following problems:

1. Only a subset of ARMv7 instructions are supported
2. Thumb-2 is unsupported
3. The code is not SMP safe

We could try to fix this code, but it turns out that because of the above issues it is rarely used in practice. GDB, for example, uses PTRACE_POKETEXT and PTRACE_PEEKTEXT to manage breakpoints itself and does not require any kernel assistance.

This patch removes the single-step emulation code from ptrace meaning that the PTRACE_SINGLESTEP request will return -EIO on ARM. Portable code must check the return value from a ptrace call and handle the failure gracefully.

Signed-off-by: Will Deacon <will.deacon at arm.com>
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The comments I received about v1 suggest that:

• If emulation is required, it is plausible to do it from userspace
• ltrace uses the SINGLESTEP call (conditionally at compile-time since other architectures, such as mips, do not support this request) but does not check the return value from ptrace. This is a bug in ltrace.
• strace does not use SINGLESTEP