* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > +Why do we need stack validation? > +-------------------------------- > + > +Here are some of the benefits of validating stack metadata: > + > +a) More reliable stack traces for frame pointer enabled kernels > + > + Frame pointers are used for debugging purposes. They allow runtime > + code and debug tools to be able to walk the stack to determine the > + chain of function call sites that led to the currently executing > + code. > + > + For some architectures, frame pointers are enabled by > + CONFIG_FRAME_POINTER. For some other architectures they may be > + required by the ABI (sometimes referred to as "backchain pointers"). > + > + For C code, gcc automatically generates instructions for setting up > + frame pointers when the -fno-omit-frame-pointer option is used. > + > + But for asm code, the frame setup instructions have to be written by > + hand, which most people don't do. So the end result is that > + CONFIG_FRAME_POINTER is honored for C code but not for most asm code. > + > + For stack traces based on frame pointers to be reliable, all > + functions which call other functions must first create a stack frame > + and update the frame pointer. If a first function doesn't properly > + create a stack frame before calling a second function, the *caller* > + of the first function will be skipped on the stack trace. > + > + The benefit of stackvalidate here is that it ensures that *all* > + functions honor CONFIG_FRAME_POINTER. As a result, no functions will > + ever [*] be skipped on a stack trace. > + > + [*] unless an interrupt or exception has occurred at the very > + beginning of a function before the stack frame has been created, > + or at the very end of the function after the stack frame has been > + destroyed. This is an inherent limitation of frame pointers. What this section does not point out is the actual effects of missing frame pointer annotations. I.e. how about quoting a before/after stack backtrace to demonstrate it? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
