--- /dev/null
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * linux/arch/arm/lib/backtrace-clang.S
+ *
+ * Copyright (C) 2019 Nathan Huckleberry
+ *
+ */
+#include <linux/kern_levels.h>
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+ .text
+
+/* fp is 0 or stack frame */
+
+#define frame r4
+#define sv_fp r5
+#define sv_pc r6
+#define mask r7
+#define sv_lr r8
+
+ENTRY(c_backtrace)
+
+#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
+ ret lr
+ENDPROC(c_backtrace)
+#else
+
+
+/*
+ * Clang does not store pc or sp in function prologues so we don't know exactly
+ * where the function starts.
+ *
+ * We can treat the current frame's lr as the saved pc and the preceding
+ * frame's lr as the current frame's lr, but we can't trace the most recent
+ * call. Inserting a false stack frame allows us to reference the function
+ * called last in the stacktrace.
+ *
+ * If the call instruction was a bl we can look at the callers branch
+ * instruction to calculate the saved pc. We can recover the pc in most cases,
+ * but in cases such as calling function pointers we cannot. In this case,
+ * default to using the lr. This will be some address in the function, but will
+ * not be the function start.
+ *
+ * Unfortunately due to the stack frame layout we can't dump r0 - r3, but these
+ * are less frequently saved.
+ *
+ * Stack frame layout:
+ * <larger addresses>
+ * saved lr
+ * frame=> saved fp
+ * optionally saved caller registers (r4 - r10)
+ * optionally saved arguments (r0 - r3)
+ * <top of stack frame>
+ * <smaller addresses>
+ *
+ * Functions start with the following code sequence:
+ * corrected pc => stmfd sp!, {..., fp, lr}
+ * add fp, sp, #x
+ * stmfd sp!, {r0 - r3} (optional)
+ *
+ *
+ *
+ *
+ *
+ *
+ * The diagram below shows an example stack setup for dump_stack.
+ *
+ * The frame for c_backtrace has pointers to the code of dump_stack. This is
+ * why the frame of c_backtrace is used to for the pc calculation of
+ * dump_stack. This is why we must move back a frame to print dump_stack.
+ *
+ * The stored locals for dump_stack are in dump_stack's frame. This means that
+ * to fully print dump_stack's frame we need both the frame for dump_stack (for
+ * locals) and the frame that was called by dump_stack (for pc).
+ *
+ * To print locals we must know where the function start is. If we read the
+ * function prologue opcodes we can determine which variables are stored in the
+ * stack frame.
+ *
+ * To find the function start of dump_stack we can look at the stored LR of
+ * show_stack. It points at the instruction directly after the bl dump_stack.
+ * We can then read the offset from the bl opcode to determine where the branch
+ * takes us. The address calculated must be the start of dump_stack.
+ *
+ * c_backtrace frame dump_stack:
+ * {[LR] } ============| ...
+ * {[FP] } =======| | bl c_backtrace
+ * | |=> ...
+ * {[R4-R10]} |
+ * {[R0-R3] } | show_stack:
+ * dump_stack frame | ...
+ * {[LR] } =============| bl dump_stack
+ * {[FP] } <=======| |=> ...
+ * {[R4-R10]}
+ * {[R0-R3] }
+ */
+
+ stmfd sp!, {r4 - r9, fp, lr} @ Save an extra register
+ @ to ensure 8 byte alignment
+ movs frame, r0 @ if frame pointer is zero
+ beq no_frame @ we have no stack frames
+ tst r1, #0x10 @ 26 or 32-bit mode?
+ moveq mask, #0xfc000003
+ movne mask, #0 @ mask for 32-bit
+
+/*
+ * Switches the current frame to be the frame for dump_stack.
+ */
+ add frame, sp, #24 @ switch to false frame
+for_each_frame: tst frame, mask @ Check for address exceptions
+ bne no_frame
+
+/*
+ * sv_fp is the stack frame with the locals for the current considered
+ * function.
+ *
+ * sv_pc is the saved lr frame the frame above. This is a pointer to a code
+ * address within the current considered function, but it is not the function
+ * start. This value gets updated to be the function start later if it is
+ * possible.
+ */
+1001: ldr sv_pc, [frame, #4] @ get saved 'pc'
+1002: ldr sv_fp, [frame, #0] @ get saved fp
+
+ teq sv_fp, mask @ make sure next frame exists
+ beq no_frame
+
+/*
+ * sv_lr is the lr from the function that called the current function. This is
+ * a pointer to a code address in the current function's caller. sv_lr-4 is
+ * the instruction used to call the current function.
+ *
+ * This sv_lr can be used to calculate the function start if the function was
+ * called using a bl instruction. If the function start can be recovered sv_pc
+ * is overwritten with the function start.
+ *
+ * If the current function was called using a function pointer we cannot
+ * recover the function start and instead continue with sv_pc as an arbitrary
+ * value within the current function. If this is the case we cannot print
+ * registers for the current function, but the stacktrace is still printed
+ * properly.
+ */
+1003: ldr sv_lr, [sv_fp, #4] @ get saved lr from next frame
+
+ ldr r0, [sv_lr, #-4] @ get call instruction
+ ldr r3, .Lopcode+4
+ and r2, r3, r0 @ is this a bl call
+ teq r2, r3
+ bne finished_setup @ give up if it's not
+ and r0, #0xffffff @ get call offset 24-bit int
+ lsl r0, r0, #8 @ sign extend offset
+ asr r0, r0, #8
+ ldr sv_pc, [sv_fp, #4] @ get lr address
+ add sv_pc, sv_pc, #-4 @ get call instruction address
+ add sv_pc, sv_pc, #8 @ take care of prefetch
+ add sv_pc, sv_pc, r0, lsl #2@ find function start
+
+finished_setup:
+
+ bic sv_pc, sv_pc, mask @ mask PC/LR for the mode
+
+/*
+ * Print the function (sv_pc) and where it was called from (sv_lr).
+ */
+1004: mov r0, sv_pc
+
+ mov r1, sv_lr
+ mov r2, frame
+ bic r1, r1, mask @ mask PC/LR for the mode
+ bl dump_backtrace_entry
+
+/*
+ * Test if the function start is a stmfd instruction to determine which
+ * registers were stored in the function prologue.
+ *
+ * If we could not recover the sv_pc because we were called through a function
+ * pointer the comparison will fail and no registers will print. Unwinding will
+ * continue as if there had been no registers stored in this frame.
+ */
+1005: ldr r1, [sv_pc, #0] @ if stmfd sp!, {..., fp, lr}
+ ldr r3, .Lopcode @ instruction exists,
+ teq r3, r1, lsr #11
+ ldr r0, [frame] @ locals are stored in
+ @ the preceding frame
+ subeq r0, r0, #4
+ bleq dump_backtrace_stm @ dump saved registers
+
+/*
+ * If we are out of frames or if the next frame is invalid.
+ */
+ teq sv_fp, #0 @ zero saved fp means
+ beq no_frame @ no further frames
+
+ cmp sv_fp, frame @ next frame must be
+ mov frame, sv_fp @ above the current frame
+ bhi for_each_frame
+
+1006: adr r0, .Lbad
+ mov r1, frame
+ bl printk
+no_frame: ldmfd sp!, {r4 - r9, fp, pc}
+ENDPROC(c_backtrace)
+ .pushsection __ex_table,"a"
+ .align 3
+ .long 1001b, 1006b
+ .long 1002b, 1006b
+ .long 1003b, 1006b
+ .long 1004b, 1006b
+ .long 1005b, 1006b
+ .popsection
+
+.Lbad: .asciz "Backtrace aborted due to bad frame pointer <%p>\n"
+ .align
+.Lopcode: .word 0xe92d4800 >> 11 @ stmfd sp!, {... fp, lr}
+ .word 0x0b000000 @ bl if these bits are set
+
+#endif
projects / kernel.git / commitdiff