backtrace: Introduce backtrace function

This function diplays the backtrace, the current EL and security state
to allow a post-processing tool to choose the right binary to interpret
the dump.

The output can be fed to GNU addr2line to resolve function names given
an ELF binary compiled with debug information. The "-i" flag is
recommended to improve display in case of inlined functions. The *.dump
files generated during the build process can also be used.

The function works in AArch64 and AArch32. In AArch32 it only works in
A32 mode (without T32 interworking), which is enforced in the Makefile.

Sample output of a backtrace at EL3:

    BACKTRACE: START: function_name
    0: EL3: 0x798
    1: EL3: 0x538
    2: EL3: 0x550
    3: EL3: 0x55c
    4: EL3: 0x568
    5: EL3: 0x5a8
    6: EL3: 0xf4
    BACKTRACE: END: function_name

In order to enable it the new option ENABLE_BACKTRACE must be set to 1.
This option is set to 1 by default only in AArch64 debug builds. As
usual, it can be overridden by the platform makefile and in the build
command line.

Change-Id: Icaff39b0e5188329728be2f3c72b868b2368e794
Co-authored-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>

1 files changed, 256 insertions, 0 deletions

diff --git a/common/backtrace.c b/common/backtrace.c
new file mode 100644
index 00000000..a91d0657
--- /dev/null
+++ b/common/backtrace.c
@@ -0,0 +1,256 @@
+/*
+ * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <arch_helpers.h>
+#include <assert.h>
+#include <console.h>
+#include <debug.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+/* Maximum number of entries in the backtrace to display */
+#define UNWIND_LIMIT	20U
+
+/*
+ * If -fno-omit-frame-pointer is used:
+ *
+ * - AArch64: The AAPCS defines the format of the frame records and mandates the
+ *   usage of r29 as frame pointer.
+ *
+ * - AArch32: The format of the frame records is not defined in the AAPCS.
+ *   However, at least GCC and Clang use the same format. When they are forced
+ *   to only generate A32 code (with -marm), they use r11 as frame pointer and a
+ *   similar format as in AArch64. If interworking with T32 is enabled, the
+ *   frame pointer is r7 and the format is  different. This is not supported by
+ *   this implementation of backtrace, so it is needed to use -marm.
+ */
+
+/* Frame records form a linked list in the stack */
+struct frame_record {
+	/* Previous frame record in the list */
+	struct frame_record *parent;
+	/* Return address of the function at this level */
+	uintptr_t return_addr;
+};
+
+static const char *get_el_str(unsigned int el)
+{
+	if (el == 3U) {
+		return "EL3";
+	} else if (el == 2U) {
+		return "EL2";
+	} else {
+		return "S-EL1";
+	}
+}
+
+/*
+ * Returns true if the address points to a virtual address that can be read at
+ * the current EL, false otherwise.
+ */
+#ifdef AARCH64
+static bool is_address_readable(uintptr_t addr)
+{
+	unsigned int el = get_current_el();
+
+	if (el == 3U) {
+		ats1e3r(addr);
+	} else if (el == 2U) {
+		ats1e2r(addr);
+	} else {
+		ats1e1r(addr);
+	}
+
+	isb();
+
+	/* If PAR.F == 1 the address translation was aborted.  */
+	if ((read_par_el1() & PAR_F_MASK) != 0U)
+		return false;
+
+	return true;
+}
+#else /* if AARCH32 */
+static bool is_address_readable(uintptr_t addr)
+{
+	unsigned int el = get_current_el();
+
+	if (el == 3U) {
+		write_ats1cpr(addr);
+	} else if (el == 2U) {
+		write_ats1hr(addr);
+	} else {
+		write_ats1cpr(addr);
+	}
+
+	isb();
+
+	/* If PAR.F == 1 the address translation was aborted.  */
+	if ((read64_par() & PAR_F_MASK) != 0U)
+		return false;
+
+	return true;
+}
+#endif
+
+/*
+ * Returns true if all the bytes in a given object are in mapped memory and an
+ * LDR using this pointer would succeed, false otherwise.
+ */
+static bool is_valid_object(uintptr_t addr, size_t size)
+{
+	assert(size > 0U);
+
+	if (addr == 0U)
+		return false;
+
+	/* Detect overflows */
+	if ((addr + size) < addr)
+		return false;
+
+	/* A pointer not aligned properly could trigger an alignment fault. */
+	if ((addr & (sizeof(uintptr_t) - 1U)) != 0U)
+		return false;
+
+	/* Check that all the object is readable */
+	for (size_t i = 0; i < size; i++) {
+		if (!is_address_readable(addr + i))
+			return false;
+	}
+
+	return true;
+}
+
+/*
+ * Returns true if the specified address is correctly aligned and points to a
+ * valid memory region.
+ */
+static bool is_valid_jump_address(uintptr_t addr)
+{
+	if (addr == 0U)
+		return false;
+
+	/* Check alignment. Both A64 and A32 use 32-bit opcodes */
+	if ((addr & (sizeof(uint32_t) - 1U)) != 0U)
+		return false;
+
+	if (!is_address_readable(addr))
+		return false;
+
+	return true;
+}
+
+/*
+ * Returns true if the pointer points at a valid frame record, false otherwise.
+ */
+static bool is_valid_frame_record(struct frame_record *fr)
+{
+	return is_valid_object((uintptr_t)fr, sizeof(struct frame_record));
+}
+
+/*
+ * Adjust the frame-pointer-register value by 4 bytes on AArch32 to have the
+ * same layout as AArch64.
+ */
+static struct frame_record *adjust_frame_record(struct frame_record *fr)
+{
+#ifdef AARCH64
+	return fr;
+#else
+	return (struct frame_record *)((uintptr_t)fr - 4U);
+#endif
+}
+
+static void unwind_stack(struct frame_record *fr, uintptr_t current_pc,
+			 uintptr_t link_register)
+{
+	uintptr_t call_site;
+	static const char *backtrace_str = "%u: %s: 0x%lx\n";
+	const char *el_str = get_el_str(get_current_el());
+
+	if (!is_valid_frame_record(fr)) {
+		printf("ERROR: Corrupted frame pointer (frame record address = %p)\n",
+		       fr);
+		return;
+	}
+
+	if (fr->return_addr != link_register) {
+		printf("ERROR: Corrupted stack (frame record address = %p)\n",
+		       fr);
+		return;
+	}
+
+	/* The level 0 of the backtrace is the current backtrace function */
+	printf(backtrace_str, 0U, el_str, current_pc);
+
+	/*
+	 * The last frame record pointer in the linked list at the beginning of
+	 * the stack should be NULL unless stack is corrupted.
+	 */
+	for (unsigned int i = 1U; i < UNWIND_LIMIT; i++) {
+		/* If an invalid frame record is found, exit. */
+		if (!is_valid_frame_record(fr))
+			return;
+		/*
+		 * A32 and A64 are fixed length so the address from where the
+		 * call was made is the instruction before the return address,
+		 * which is always 4 bytes before it.
+		 */
+		call_site = fr->return_addr - 4U;
+
+		/*
+		 * If the address is invalid it means that the frame record is
+		 * probably corrupted.
+		 */
+		if (!is_valid_jump_address(call_site))
+			return;
+
+		printf(backtrace_str, i, el_str, call_site);
+
+		fr = adjust_frame_record(fr->parent);
+	}
+
+	printf("ERROR: Max backtrace depth reached\n");
+}
+
+/*
+ * Display a backtrace. The cookie string parameter is displayed along the
+ * trace to help filter the log messages.
+ *
+ * Many things can prevent displaying the expected backtrace. For example,
+ * compiler optimizations can use a branch instead of branch with link when it
+ * detects a tail call. The backtrace level for this caller will not be
+ * displayed, as it does not appear in the call stack anymore. Also, assembly
+ * functions will not be displayed unless they setup AAPCS compliant frame
+ * records on AArch64 and compliant with GCC-specific frame record format on
+ * AArch32.
+ *
+ * Usage of the trace: addr2line can be used to map the addresses to function
+ * and source code location when given the ELF file compiled with debug
+ * information. The "-i" flag is highly recommended to improve display of
+ * inlined function. The *.dump files generated when buildidng each image can
+ * also be used.
+ *
+ * WARNING: In case of corrupted stack, this function could display security
+ * sensitive information past the beginning of the stack so it must not be used
+ * in production build. This function is only compiled in when ENABLE_BACKTRACE
+ * is set to 1.
+ */
+void backtrace(const char *cookie)
+{
+	uintptr_t return_address = (uintptr_t)__builtin_return_address(0U);
+	struct frame_record *fr = __builtin_frame_address(0U);
+
+	/* Printing the backtrace may crash the system, flush before starting */
+	(void)console_flush();
+
+	fr = adjust_frame_record(fr);
+
+	printf("BACKTRACE: START: %s\n", cookie);
+
+	unwind_stack(fr, (uintptr_t)&backtrace, return_address);
+
+	printf("BACKTRACE: END: %s\n", cookie);
+}