riscv: ftrace: Reduce the detour code size to half
Use a temporary register to reduce the size of detour code from 16 bytes to 8 bytes. The previous implementation is from 'commit afc76b8b ("riscv: Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")'. Before the patch: <func_prolog>: 0: REG_S ra, -SZREG(sp) 4: auipc ra, ? 8: jalr ?(ra) 12: REG_L ra, -SZREG(sp) (func_boddy) After the patch: <func_prolog>: 0: auipc t0, ? 4: jalr t0, ?(t0) (func_boddy) This patch not just reduces the size of detour code, but also fixes an important issue: An Ftrace callback registered with FTRACE_OPS_FL_IPMODIFY flag can actually change the instruction pointer, e.g. to "replace" the given kernel function with a new one, which is needed for livepatching, etc. In this case, the trampoline (ftrace_regs_caller) would not return to <func_prolog+12> but would rather jump to the new function. So, "REG_L ra, -SZREG(sp)" would not run and the original return address would not be restored. The kernel is likely to hang or crash as a result. This can be easily demonstrated if one tries to "replace", say, cmdline_proc_show() with a new function with the same signature using instruction_pointer_set(&fregs->regs, new_func_addr) in the Ftrace callback. Link: https://lore.kernel.org/linux-riscv/20221122075440.1165172-1-suagrfillet@gmail.com/ Link: https://lore.kernel.org/linux-riscv/d7d5730b-ebef-68e5-5046-e763e1ee6164@yadro.com/Co-developed-by: Song Shuai <suagrfillet@gmail.com> Signed-off-by: Song Shuai <suagrfillet@gmail.com> Signed-off-by: Guo Ren <guoren@linux.alibaba.com> Signed-off-by: Guo Ren <guoren@kernel.org> Cc: Evgenii Shatokhin <e.shatokhin@yadro.com> Reviewed-by: Evgenii Shatokhin <e.shatokhin@yadro.com> Link: https://lore.kernel.org/r/20230112090603.1295340-4-guoren@kernel.org Cc: stable@vger.kernel.org Fixes: 10626c32 ("riscv/ftrace: Add basic support") Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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