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This patch implements FTRACE_WITH_REGS for arm64, which allows a traced function's arguments (and some other registers) to be captured into a struct pt_regs, allowing these to be inspected and/or modified. This is a building block for live-patching, where a function's arguments may be forwarded to another function. This is also necessary to enable ftrace and in-kernel pointer authentication at the same time, as it allows the LR value to be captured and adjusted prior to signing. Using GCC's -fpatchable-function-entry=N option, we can have the compiler insert a configurable number of NOPs between the function entry point and the usual prologue. This also ensures functions are AAPCS compliant (e.g. disabling inter-procedural register allocation). For example, with -fpatchable-function-entry=2, GCC 8.1.0 compiles the following: | unsigned long bar(void); | | unsigned long foo(void) | { | return bar() + 1; | } ... to: | <foo>: | nop | nop | stp x29, x30, [sp, #-16]! | mov x29, sp | bl 0 <bar> | add x0, x0, #0x1 | ldp x29, x30, [sp], #16 | ret This patch builds the kernel with -fpatchable-function-entry=2, prefixing each function with two NOPs. To trace a function, we replace these NOPs with a sequence that saves the LR into a GPR, then calls an ftrace entry assembly function which saves this and other relevant registers: | mov x9, x30 | bl <ftrace-entry> Since patchable functions are AAPCS compliant (and the kernel does not use x18 as a platform register), x9-x18 can be safely clobbered in the patched sequence and the ftrace entry code. There are now two ftrace entry functions, ftrace_regs_entry (which saves all GPRs), and ftrace_entry (which saves the bare minimum). A PLT is allocated for each within modules. Signed-off-by: Torsten Duwe <[email protected]> [Mark: rework asm, comments, PLTs, initialization, commit message] Signed-off-by: Mark Rutland <[email protected]> Reviewed-by: Amit Daniel Kachhap <[email protected]> Reviewed-by: Ard Biesheuvel <[email protected]> Reviewed-by: Torsten Duwe <[email protected]> Tested-by: Amit Daniel Kachhap <[email protected]> Tested-by: Torsten Duwe <[email protected]> Cc: AKASHI Takahiro <[email protected]> Cc: Catalin Marinas <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: Julien Thierry <[email protected]> Cc: Will Deacon <[email protected]>
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Torsten Duwe
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Mark Rutland
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Nov 6, 2019
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