MIPS: Calculate microMIPS ra properly when unwinding the stack
get_frame_info() calculates the offset of the return address within a stack frame simply by dividing a the bottom 16 bits of the instruction, treated as a signed integer, by the size of a long. Whilst this works for MIPS32 & MIPS64 ISAs where the sw or sd instructions are used, it's incorrect for microMIPS where encodings differ. The result is that we typically completely fail to unwind the stack on microMIPS. Fix this by adjusting is_ra_save_ins() to calculate the return address offset, and take into account the various different encodings there in the same place as we consider whether an instruction is storing the ra/$31 register. With this we are now able to unwind the stack for kernels targetting the microMIPS ISA, for example we can produce: Call Trace: [<80109e1f>] show_stack+0x63/0x7c [<8011ea17>] __warn+0x9b/0xac [<8011ea45>] warn_slowpath_fmt+0x1d/0x20 [<8013fe53>] register_console+0x43/0x314 [<8067c58d>] of_setup_earlycon+0x1dd/0x1ec [<8067f63f>] early_init_dt_scan_chosen_stdout+0xe7/0xf8 [<8066c115>] do_early_param+0x75/0xac [<801302f9>] parse_args+0x1dd/0x308 [<8066c459>] parse_early_options+0x25/0x28 [<8066c48b>] parse_early_param+0x2f/0x38 [<8066e8cf>] setup_arch+0x113/0x488 [<8066c4f3>] start_kernel+0x57/0x328 ---[ end trace 0000000000000000 ]--- Whereas previously we only produced: Call Trace: [<80109e1f>] show_stack+0x63/0x7c ---[ end trace 0000000000000000 ]--- Signed-off-by: Paul Burton <paul.burton@imgtec.com> Fixes: 34c2f668 ("MIPS: microMIPS: Add unaligned access support.") Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com> Cc: linux-mips@linux-mips.org Cc: <stable@vger.kernel.org> # v3.10+ Patchwork: https://patchwork.linux-mips.org/patch/14532/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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