[PATCH] doc: describe the analysis of crash dumps
Heinrich Schuchardt
xypron.glpk at gmx.de
Fri Apr 24 23:31:20 CEST 2020
Provide an overview of the analysis of U-Boot crash dumps.
Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
---
doc/develop/crash_dumps.rst | 122 ++++++++++++++++++++++++++++++++++++
doc/develop/index.rst | 10 +++
doc/index.rst | 11 ++++
3 files changed, 143 insertions(+)
create mode 100644 doc/develop/crash_dumps.rst
create mode 100644 doc/develop/index.rst
diff --git a/doc/develop/crash_dumps.rst b/doc/develop/crash_dumps.rst
new file mode 100644
index 0000000000..5aede4a9af
--- /dev/null
+++ b/doc/develop/crash_dumps.rst
@@ -0,0 +1,122 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (c) 2020 Heinrich Schuchardt
+
+Analyzing crash dumps
+=====================
+
+When the CPU detects an instruction that it cannot execute it raises an
+interrupt. U-Boot than writes a crash dump. This chapter describes how such
+dump can be analyzed.
+
+Creating a crash dump voluntarily
+---------------------------------
+
+For describing the analysis of a crash dump we need an example. U-Boot comes
+with a command 'exception' that comes in handy here. The command is enabled
+by::
+
+ CONFIG_CMD_EXCEPTION=y
+
+The example output below was recorded when running qemu\_arm64\_defconfig on
+QEMU::
+
+ => exception undefined
+ "Synchronous Abort" handler, esr 0x02000000
+ elr: 00000000000101fc lr : 00000000000214ec (reloc)
+ elr: 000000007ff291fc lr : 000000007ff3a4ec
+ x0 : 000000007ffbd7f8 x1 : 0000000000000000
+ x2 : 0000000000000001 x3 : 000000007eedce18
+ x4 : 000000007ff291fc x5 : 000000007eedce50
+ x6 : 0000000000000064 x7 : 000000007eedce10
+ x8 : 0000000000000000 x9 : 0000000000000004
+ x10: 6db6db6db6db6db7 x11: 000000000000000d
+ x12: 0000000000000006 x13: 000000000001869f
+ x14: 000000007edd7dc0 x15: 0000000000000002
+ x16: 000000007ff291fc x17: 0000000000000000
+ x18: 000000007eed8dc0 x19: 0000000000000000
+ x20: 000000007ffbd7f8 x21: 0000000000000000
+ x22: 000000007eedce10 x23: 0000000000000002
+ x24: 000000007ffd4c80 x25: 0000000000000000
+ x26: 0000000000000000 x27: 0000000000000000
+ x28: 000000007eedce70 x29: 000000007edd7b40
+
+ Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)
+ Resetting CPU ...
+
+ resetting ...
+
+The first line provides us with the type of interrupt that occurred.
+(On ARMv8 a synchronous abort is an exception where the return address stored
+in the ESR register indicates the instruction that caused the exception.)
+
+The second line provides the contents of the elr and the lr register after
+subtracting the relocation offset. - U-Boot relocates itself after being
+loaded. - The relocation offset can also be displayed using the bdinfo command.
+
+After the contents of the registers we get a line indicating the machine
+code of the instructions preceding the crash and in parentheses the instruction
+leading to the dump.
+
+Analyzing the code location
+---------------------------
+
+We can convert the instructions in the line starting with 'Code:' into mnemonics
+using the objdump command. To make things easier scripts/decodecode is
+supplied::
+
+ $echo 'Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)' | \
+ CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 scripts/decodecode
+ Code: b00003c0 912ad000 940029d6 17ffff52 (e7f7defb)
+ All code
+ ========
+ 0: b00003c0 adrp x0, 0x79000
+ 4: 912ad000 add x0, x0, #0xab4
+ 8: 940029d6 bl 0xa760
+ c: 17ffff52 b 0xfffffffffffffd54
+ 10:* e7f7defb .inst 0xe7f7defb ; undefined <-- trapping instruction
+
+ Code starting with the faulting instruction
+ ===========================================
+ 0: e7f7defb .inst 0xe7f7defb ; undefined
+
+Now lets use the locations provided by the elr and lr registers after
+subtracting the relocation offset to find out where in the code the crash
+occurred and from where it was invoked.
+
+File u-boot.map contains the memory layout of the U-Boot binary. Here we find
+these lines::
+
+ .text.do_undefined
+ 0x00000000000101fc 0xc cmd/built-in.o
+ .text.exception_complete
+ 0x0000000000010208 0x90 cmd/built-in.o
+ ...
+ .text.cmd_process
+ 0x00000000000213b8 0x164 common/built-in.o
+ 0x00000000000213b8 cmd_process
+ .text.cmd_process_error
+ 0x000000000002151c 0x40 common/built-in.o
+ 0x000000000002151c cmd_process_error
+
+So the error occurred at the start of function do\_undefined() and this
+function was invoked from somewhere inside function cmd\_process().
+
+If we want to dive deeper, we can disassemble the U-Boot binary::
+
+ $ aarch64-linux-gnu-objdump -S -D u-boot | less
+
+ 00000000000101fc <do_undefined>:
+ {
+ /*
+ * 0xe7f...f. is undefined in ARM mode
+ * 0xde.. is undefined in Thumb mode
+ */
+ asm volatile (".word 0xe7f7defb\n");
+ 101fc: e7f7defb .inst 0xe7f7defb ; undefined
+ return CMD_RET_FAILURE;
+ }
+ 10200: 52800020 mov w0, #0x1 // #1
+ 10204: d65f03c0 ret
+
+This example is based on the ARMv8 architecture but the same procedures can be
+used on other architectures as well.
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
new file mode 100644
index 0000000000..072db63b5c
--- /dev/null
+++ b/doc/develop/index.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Develop U-Boot
+==============
+
+
+.. toctree::
+ :maxdepth: 2
+
+ crash_dumps
diff --git a/doc/index.rst b/doc/index.rst
index cd98be6cc5..fd9f10f28e 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -26,6 +26,17 @@ trying to get it to work optimally on a given system.
build/index
+Developer-oriented documentation
+--------------------------------
+
+The following manuals are written for *developers* of the U-Boot - those who
+want to contribute to U-Boot.
+
+.. toctree::
+ :maxdepth: 2
+
+ develop/index
+
Unified Extensible Firmware (UEFI)
----------------------------------
--
2.26.2
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