bak

README.md

@@ -384,6 +384,15 @@ And then tell GDB where the module was loaded with:
     Ctrl + C
     add-symbol-file ../kernel_module-1.0/fops.ko 0xfffffffa00000000
 
+### Debug kernel early boot
+
+TODO: why can't we break at early startup stuff such as:
+
+    ./rungdb extract_kernel
+    ./rungdb main
+
+See also: <https://stackoverflow.com/questions/2589845/what-are-the-first-operations-that-the-linux-kernel-executes-on-boot>
+
 ## Other architectures
 
 The portability of the kernel and toolchains is amazing: change an option and most things magically work on completely different hardware.
@@ -602,6 +611,29 @@ which automatically finds unstripped shared libraries on the host for us.
 
 See also: <https://stackoverflow.com/questions/8611194/debugging-shared-libraries-with-gdbserver/45252113#45252113>
 
+### Debug userland process directly from QEMU
+
+GDB breakpoints are set on virtual addresses, so you can in theory debug userland processes as well.
+
+<https://stackoverflow.com/questions/26271901/is-it-possible-to-use-gdb-and-qemu-to-debug-linux-user-space-programs-and-kernel>
+
+    ./runqemu -d -e 'init=/rand_check.out' -n
+
+On another shell:
+
+    buildroot/output.x86_64~/host/usr/bin/x86_64-linux-readelf -h buildroot/output.x86_64~/build/kernel_module-1.0/user/rand_check.out | grep Entry
+    # Entry point address:               0x400560
+    buildroot/output.x86_64~/host/usr/bin/x86_64-linux-readelf -s buildroot/output.x86_64~/build/kernel_module-1.0/user/rand_check.out | grep -E '\bmain\b'
+    # 68: 0000000000400748   309 FUNC    GLOBAL DEFAULT    8 main
+    ./rungdb '*0x400748'
+
+Alternatively, from inside GDB you can do the more succinct:
+
+    shell ../../host/usr/bin/x86_64-linux-readelf -h ../kernel_module-1.0/user/rand_check.out | grep Ent
+    shell ../../host/usr/bin/x86_64-linux-readelf -s ../kernel_module-1.0/user/rand_check.out | grep -E '\bmain\b'
+
+Those steps should be fully automatable `.gdbinit` script.
+
 ## X11
 
 Only tested successfully in `x86_64`:

rungdb

@@ -2,13 +2,20 @@
 
 set -e
 
-arch=x86_64
+arch='x86_64'
+bdfore=''
 kgdb=false
-while getopts a:k OPT; do
+while getopts A:a:b:k OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
       ;;
+    A)
+      after="$OPTARG"
+      ;;
+    b)
+      before="$OPTARG"
+      ;;
     k)
       kgdb=true
       ;;
@@ -17,13 +24,14 @@ done
 shift "$(($OPTIND - 1))"
 if [ "$#" -gt 0 ]; then
   brk="-ex 'break $1'"
+  shift
 else
   brk=''
 fi
 
 buildroot_out_dir="$(pwd)/buildroot/output.${arch}~"
-gdb="${buildroot_out_dir}/host/usr/bin/${arch}-linux-gdb"
-cd "${buildroot_out_dir}/build"/linux-custom/
+gdb="${buildroot_out_dir}/host/usr/bin/${arch}-linux-gdb $before"
+cd "${buildroot_out_dir}/build/linux-custom/"
 if "$kgdb"; then
     cmd="$gdb \
       -q \
@@ -48,7 +56,7 @@ else
         -ex 'disconnect' \
         -ex 'set arch i386:x86-64' \
         -ex 'target remote localhost:1234' \
-        -ex 'lx-symbols ../kernel_module-1.0/'
+        -ex 'lx-symbols ../kernel_module-1.0/' \
       "
     ;;
     'arm'|'aarch64'|'mips64')
@@ -63,5 +71,6 @@ else
     ;;
   esac
 fi
+cmd="$cmd $after"
 echo "$cmd"
 eval "$cmd"