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Pervasive Debugging
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===================
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040205 Alex Ho (alex.ho@cl.cam.ac.uk)
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Introduction
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------------
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The pervasive debugging project is leveraging Xen to
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debug distributed systems. We have added a gdb stub
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to Xen to allow for remote debugging of both Xen and
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guest operating systems. More information about the
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pervasive debugger is available at: http://www.cl.cam.ac.uk/netos/pdb
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Implementation
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--------------
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The gdb stub communicates with gdb running over a serial line.
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The main entry point is pdb_handle_exception() which is invoked
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from: pdb_key_pressed() ('D' on the console)
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do_int3_exception() (interrupt 3: breakpoint exception)
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do_debug() (interrupt 1: debug exception)
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This accepts characters from the serial port and passes gdb
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commands to pdb_process_command() which implements the gdb stub
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interface. This file draws heavily from the kgdb project and
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sample gdbstub provided with gdb.
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The stub can examine registers, single step and continue, and
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read and write memory (in Xen, a domain, or a Linux process'
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address space). The debugger does not currently trace the
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current process, so all bets are off if context switch occurs
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in the domain.
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Setup
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-----
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+-------+ telnet +-----------+ serial +-------+
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| GDB |--------| nsplitd |--------| Xen |
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+-------+ +-----------+ +-------+
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To run pdb, Xen must be appropriately configured and
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a suitable serial interface attached to the target machine.
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GDB and nsplitd can run on the same machine.
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Xen Configuration
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Add the "pdb=xxx" option to your Xen boot command line
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where xxx is one of the following values:
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com1 gdb stub should communicate on com1
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com1H gdb stub should communicate on com1 (with high bit set)
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com2 gdb stub should communicate on com2
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com2H gdb stub should communicate on com2 (with high bit set)
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Symbolic debugging infomration is quite helpful too:
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xeno.bk/xen/arch/i386/Rules.mk
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add -g to CFLAGS to compile Xen with symbols
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xeno.bk/xenolinux-2.4.24-sparse/arch/xeno/Makefile
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add -g to CFLAGS to compile XenoLinux with symbols
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You may also want to consider dedicating a register to the
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frame pointer (disable the -fomit-frame-pointer compile flag).
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When booting Xen and domain 0, look for the console text
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"Initializing pervasive debugger (PDB)" just before DOM0 starts up.
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Serial Port Configuration
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pdb expects to communicate with gdb using the serial port. Since
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this port is often shared with the machine's console output, pdb can
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discriminate its communication by setting the high bit of each char.
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A new tool has been added to the source tree which splits
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the serial output from a remote machine into two streams:
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one stream (without the high bit) is the console and
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one stream (with the high bit stripped) is the pdb communication.
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See: xeno.bk/tools/nsplitd
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Note: nsplitd was originally written for the Nemesis project
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at Cambridge.
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Usage:
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%telnet <hostname> <port>
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This is the console of the remote machine. You will probably
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want to set telnet in char mode (or create a .telnetrc file).
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<hostname> and <port> are for your instance of nsplitd.
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You should be able to press 'h' to display a list of keyboard
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handlers. In particular, you should see:
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key 'D' (ascii '44') => enter pervasive debugger
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After nsplitd accepts a connection on <port>, it starts listening
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on port <port + 1>. Characters sent to the <port + 1> will have the
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high bit set and vice versa for characters received.
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Note: if you are not using a serial console and can dedicate your
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serial line to pdb messages, then edit pdb_put_char and pdb_get_char
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in pdb-stub.c to remove the high bit tests.
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GDB 6.0
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pdb has been tested with gdb 6.0. It should also work with
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earlier versions.
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Usage
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-----
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1. Boot Xen and XenoLinux
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2. Interrupt Xen by pressing 'D' at the console
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You should see the console message:
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pdb_handle_exception [0x88][0xfc5c9d88]
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At this point Xen is waiting for gdb commands on the serial line.
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3. Attach with gdb
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(gdb) file xeno.bk/xen/xen
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Reading symbols from xeno.bk/xen/xen...done.
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(gdb) target remote <hostname>:<port + 1> /* contact nsplitd */
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Remote debugging using serial.srg:12131
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continue_cpu_idle_loop () at current.h:10
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warning: shared library handler failed to enable breakpoint
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(gdb) break __enter_scheduler
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Breakpoint 1 at 0xfc510a94: file schedule.c, line 330.
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(gdb) cont
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Continuing.
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Program received signal SIGTRAP, Trace/breakpoint trap.
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__enter_scheduler () at schedule.c:330
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(gdb) step
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(gdb) step
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(gdb) print next /* the variable prev has been optimized away! */
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$1 = (struct task_struct *) 0x0
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(gdb) delete
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Delete all breakpoints? (y or n) y
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4. You can add additional symbols to gdb
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(gdb) add-sym xenolinux-2.4.24/vmlinux
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add symbol table from file "xenolinux-2.4.24/vmlinux" at
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(y or n) y
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Reading symbols from xenolinux-2.4.24/vmlinux...done.
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(gdb) x/s cpu_vendor_names[0]
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0xc01530d2 <cpdext+62898>: "Intel"
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(gdb) break free_uid
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Breakpoint 2 at 0xc0012250
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(gdb) cont
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Continuing. /* run a command in domain 0 */
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Program received signal SIGTRAP, Trace/breakpoint trap.
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free_uid (up=0xbffff738) at user.c:77
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(gdb) print *up
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$2 = {__count = {counter = 0}, processes = {counter = 135190120}, files = {
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counter = 0}, next = 0x395, pprev = 0xbffff878, uid = 134701041}
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(gdb) finish
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Run till exit from #0 free_uid (up=0xbffff738) at user.c:77
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Program received signal SIGTRAP, Trace/breakpoint trap.
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release_task (p=0xc2da0000) at exit.c:51
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(gdb) print *p
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$3 = {state = 4, flags = 4, sigpending = 0, addr_limit = {seg = 3221225472},
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exec_domain = 0xc016a040, need_resched = 0, ptrace = 0, lock_depth = -1,
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counter = 1, nice = 0, policy = 0, mm = 0x0, processor = 0,
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cpus_runnable = 1, cpus_allowed = 4294967295, run_list = {next = 0x0,
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prev = 0x0}, sleep_time = 18995, next_task = 0xc017c000,
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prev_task = 0xc2f94000, active_mm = 0x0, local_pages = {next = 0xc2da0054,
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prev = 0xc2da0054}, allocation_order = 0, nr_local_pages = 0,
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binfmt = 0xc016c6a0, exit_code = 0, exit_signal = 17, pdeath_signal = 0,
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personality = 0, did_exec = -1, task_dumpable = 1, pid = 917, pgrp = 914,
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tty_old_pgrp = 0, session = 914, tgid = 917, leader = 0,
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p_opptr = 0xc2f94000, p_pptr = 0xc2f94000, p_cptr = 0x0, p_ysptr = 0x0,
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p_osptr = 0x0, thread_group = {next = 0xc2da00a8, prev = 0xc2da00a8},
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pidhash_next = 0x0, pidhash_pprev = 0xc01900b8, wait_chldexit = {
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lock = <incomplete type>, task_list = {next = 0xc2da00b8,
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prev = 0xc2da00b8}}, vfork_done = 0x0, rt_priority = 0,
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it_real_value = 0, it_prof_value = 0, it_virt_value = 0, it_real_incr = 0,
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it_prof_incr = 0, it_virt_incr = 0, real_timer = {list = {next = 0x0,
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prev = 0x0}, expires = 18950, data = 3269066752,
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function = 0xc000ce30 <it_real_fn>}, times = {tms_utime = 0,
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tms_stime = 0, tms_cutime = 0, tms_cstime = 0}, start_time = 18989,
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per_cpu_utime = {1}, per_cpu_stime = {310}, min_flt = 13, maj_flt = 104,
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nswap = 0, cmin_flt = 0, cmaj_flt = 0, cnswap = 0, swappable = -1, uid = 0,
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euid = 0, suid = 0, fsuid = 0, gid = 0, egid = 0, sgid = 0, fsgid = 0,
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ngroups = 7, groups = {0, 1, 2, 3, 4, 6, 10, 0 <repeats 25 times>},
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cap_effective = 4294967039, cap_inheritable = 0, cap_permitted = 4294967039,
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keep_capabilities = 0, user = 0xc016b18c, rlim = {{rlim_cur = 4294967295,
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rlim_max = 4294967295}, {rlim_cur = 4294967295, rlim_max = 4294967295}, {
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rlim_cur = 4294967295, rlim_max = 4294967295}, {rlim_cur = 8388608,
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rlim_max = 4294967295}, {rlim_cur = 0, rlim_max = 4294967295}, {
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rlim_cur = 4294967295, rlim_max = 4294967295}, {rlim_cur = 512,
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rlim_max = 512}, {rlim_cur = 1024, rlim_max = 1024}, {
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rlim_cur = 4294967295, rlim_max = 4294967295}, {rlim_cur = 4294967295,
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rlim_max = 4294967295}, {rlim_cur = 4294967295, rlim_max = 4294967295}},
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used_math = 0, comm = "id\000h\000og\000\000\000\000\000\000\000\000",
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link_count = 0, total_link_count = 1, tty = 0xc3ed1000, locks = 0,
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semundo = 0x0, semsleeping = 0x0, thread = {esp0 = 3269074944,
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eip = 3221249046, esp = 3269074792, fs = 0, gs = 0, io_pl = 3, debugreg = {
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0, 0, 0, 0, 0, 0, 0, 0}, cr2 = 0, trap_no = 0, error_code = 0, i387 = {
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fsave = {cwd = 2098047, swd = 125632512, twd = 1073944696, fip = 2091,
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fcs = -1073745032, foo = 2099, fos = 8064, st_space = {
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0 <repeats 20 times>}, status = 0}, fxsave = {cwd = 895, swd = 32,
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twd = 0, fop = 1917, fip = 1073944696, fcs = 2091, foo = -1073745032,
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fos = 2099, mxcsr = 8064, reserved = 0, st_space = {
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0 <repeats 24 times>, 1449431204, -1774489361, 16383, 0, 1,
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-1891252224, 16404, 0}, xmm_space = {0 <repeats 32 times>},
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padding = {0 <repeats 56 times>}}, soft = {cwd = 2098047,
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swd = 125632512, twd = 1073944696, fip = 2091, fcs = -1073745032,
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foo = 2099, fos = 8064, st_space = {0 <repeats 20 times>},
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ftop = 0 '\0', changed = 0 '\0', lookahead = 0 '\0',
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no_update = 0 '\0', rm = 0 '\0', alimit = 0 '\0', info = 0x0,
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entry_eip = 0}}, vm86_info = 0x0, screen_bitmap = 0, v86flags = 0,
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v86mask = 0, saved_esp0 = 0}, fs = 0x0, files = 0x0, namespace = 0x0,
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sigmask_lock = <incomplete type>, sig = 0x0, blocked = {sig = {0, 0}},
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pending = {head = 0x0, tail = 0xc2da04f8, signal = {sig = {0, 0}}},
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sas_ss_sp = 0, sas_ss_size = 0, notifier = 0, notifier_data = 0x0,
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notifier_mask = 0x0, parent_exec_id = 7, self_exec_id = 8,
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alloc_lock = <incomplete type>, journal_info = 0x0}
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