[PATCH] Fix ARCH=um segfault on x86-64.

[Rob Landley <rob@landley.net>]

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Apparently, nobody other than me has ever attempted to use User Mode Linux built from 2.6.28 on x86-64, because it doesn't work.  It still doesn't work in current git.  I complained about it not working back in January:
http://sourceforge.net/mailarchive/forum.php?thread_name=200901130159.04389.rob%40landley.net&forum_name=user-mode-linux-develhttp://lkml.indiana.edu/hypermail/linux/kernel/0901.2/00669.html
And today, I bothered to track down why.
This is the commit that broke it, when Peter Anvin merged x86 and x86-64 for ARCH=um: http://kernel.org/hg/linux-2.6/rev/117978
Here's a patch that fixes it for me:
Signed-off-by: Rob Landley <rob@landley.net>
diff -r 178a096e9e38 arch/um/Kconfig.x86--- a/arch/um/Kconfig.x86	Fri Feb 27 16:49:46 2009 -0800+++ b/arch/um/Kconfig.x86	Thu Mar 05 23:35:55 2009 -0600@@ -26,9 +26,8 @@ 	def_bool !X86_XADD  config 3_LEVEL_PGTABLES-	bool "Three-level pagetables (EXPERIMENTAL)" if !64BIT+	bool 	default 64BIT-	depends on EXPERIMENTAL 	help 	Three-level pagetables will let UML have more than 4G of physical 	memory.  All the memory that can't be mapped directly will be treated\0What changed is that the resulting .config no longer contains the line "CONFIG_3_LEVEL_PGTABLES=y" (it's not visible, and thus not written out into the config file file).  Without that symbol defined, x86-64 dies trying to boot.  If you tweak the Kconfig so the symbol gets written out, it starts working again.
I have no idea how ANYBODY has EVER managed to use 2.6.28 User Mode Linux on an x86-64 host.  My theory is that nobody ever did.  I suspect that very few people use UML anymore now that KVM and the rustyvisor and such are available, and those legacy users still fiddling with it are apparently all either using old versions or 32-bit hosts.  (I still like being able to stick printfs into the kernel.)
Here's the panic, in case you're wondering:
$ ./linux rw init=/bin/bash rootfstype=hostfsCore dump limits :	soft - 0	hard - NONEChecking that ptrace can change system call numbers...OKChecking syscall emulation patch for ptrace...OKChecking advanced syscall emulation patch for ptrace...OKChecking for tmpfs mount on /dev/shm...OKChecking PROT_EXEC mmap in /dev/shm/...OKChecking for the skas3 patch in the host:  - /proc/mm...not found: No such file or directory  - PTRACE_FAULTINFO...not found  - PTRACE_LDT...not foundUML running in SKAS0 modeAdding 4390912 bytes to physical memory to account for exec-shield gapLinux version 2.6.29-rc7 (landley@driftwood) (gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu11) ) #1 Thu Mar 5 21:20:14 CST 2009Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 9137Kernel command line: rw init=/bin/bash rootfstype=hostfs root=98:0PID hash table entries: 256 (order: 8, 2048 bytes)Dentry cache hash table entries: 8192 (order: 4, 65536 bytes)Inode-cache hash table entries: 4096 (order: 3, 32768 bytes)Memory: 29244k availableSLUB: Genslabs=12, HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1Calibrating delay loop... 209.30 BogoMIPS (lpj=1046528)Mount-cache hash table entries: 256Checking that host ptys support output SIGIO...YesChecking that host ptys support SIGIO on close...No, enabling workaroundUsing 2.6 host AIObio: create slab <bio-0> at 0Switched to NOHz mode on CPU #0io scheduler noop registered (default)loop: module loadedInitialized stdio console driverUsing a channel type which is configured out of UMLparse_chan_pair failed for device 1 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 2 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 3 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 4 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 5 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 6 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 7 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 8 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 9 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 10 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 11 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 12 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 13 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 14 : Configuration failedUsing a channel type which is configured out of UMLparse_chan_pair failed for device 15 : Configuration failedConsole initialized on /dev/tty0console [tty0] enabledVFS: Mounted root (hostfs filesystem) on device 0:8.IRQ 3/console-write: IRQF_DISABLED is not guaranteed on shared IRQsIRQ 2/console: IRQF_DISABLED is not guaranteed on shared IRQsIRQ 10/winch: IRQF_DISABLED is not guaranteed on shared IRQs
Pid: 1, comm: swapper Not tainted 2.6.29-rc7RIP: 0033:[<000000006001b342>]RSP: 0000000062029dd0  EFLAGS: 00010216RAX: 00000000622af800 RBX: 00000000621b0000 RCX: 0000000003ffc09fRDX: fffffffffff02800 RSI: 0000000060313900 RDI: 00000000622af800RBP: 0000000060d10048 R08: 0000000000000000 R09: 0000000000100000R10: 0000000000000000 R11: 0000000060197a00 R12: 000000006211f300R13: 000000006211f300 R14: 0000000060206440 R15: 0000000062020300Call Trace: 602058f8:  [<600160c5>] timer_one_shot+0x55/0x8060205908:  [<6000e4b9>] segv+0x2a9/0x2d060205918:  [<6001b342>] __memcpy+0xe/0xac60205928:  [<6003f376>] tick_dev_program_event+0x36/0xb060205958:  [<6003f5c4>] tick_check_oneshot_change+0xf4/0x10060205968:  [<6002bc6d>] run_timer_softirq+0x1cd/0x210602059e8:  [<6000e530>] segv_handler+0x50/0xe060205a08:  [<6003f250>] tick_handle_periodic+0x10/0x6060205a48:  [<60026a7d>] do_softirq+0x4d/0x7060205a68:  [<60026bf2>] irq_exit+0x42/0xa060205a88:  [<6000aecf>] do_IRQ+0x2f/0x5060205aa8:  [<600154e4>] sig_handler_common+0x64/0xe060205b30:  [<6001b342>] __memcpy+0xe/0xac60205b50:  [<600ae3de>] sysfs_new_dirent+0xfe/0x12060205bd8:  [<600156aa>] sig_handler+0x1a/0x4060205be8:  [<60015983>] handle_signal+0x73/0xb060205c28:  [<60100140>] __restore_rt+0x0/0x1060205cd8:  [<6001b342>] __memcpy+0xe/0xac
Kernel panic - not syncing: Segfault with no mm
Pid: 1, comm: swapper Not tainted 2.6.29-rc7RIP: 0033:[<00000000601003a7>]RSP: 00007fff8026e2a8  EFLAGS: 00000246RAX: 0000000000000000 RBX: 0000000000001cc4 RCX: ffffffffffffffffRDX: 0000000000000000 RSI: 0000000000000013 RDI: 0000000000001cc4RBP: 0000000000001cc0 R08: 00007fff8026e1f0 R09: 0000000000000000R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff8026e3b8R13: 0000000000000004 R14: 00007fff8026e580 R15: 00007fff8026e414Call Trace: 602057b8:  [<6003aacd>] up+0x1d/0x50602057c8:  [<6000e77d>] panic_exit+0x2d/0x50602057d8:  [<600214ac>] release_console_sem+0x19c/0x1e0602057e8:  [<6003ac87>] notifier_call_chain+0x37/0x7060205818:  [<60167cd7>] panic+0xd0/0x16460205858:  [<60100370>] __sigprocmask+0x10/0x4060205878:  [<60167df6>] printk+0x8b/0x9560205898:  [<6001604e>] os_nsecs+0xe/0x30602058b8:  [<6001b342>] __memcpy+0xe/0xac602058c8:  [<6000d080>] show_trace+0x60/0xc0602058e8:  [<6001b148>] show_regs+0x28/0x3060205908:  [<6000e4c5>] segv+0x2b5/0x2d060205918:  [<6001b342>] __memcpy+0xe/0xac60205928:  [<6003f376>] tick_dev_program_event+0x36/0xb060205958:  [<6003f5c4>] tick_check_oneshot_change+0xf4/0x10060205968:  [<6002bc6d>] run_timer_softirq+0x1cd/0x210602059e8:  [<6000e530>] segv_handler+0x50/0xe060205a08:  [<6003f250>] tick_handle_periodic+0x10/0x6060205a48:  [<60026a7d>] do_softirq+0x4d/0x7060205a68:  [<60026bf2>] irq_exit+0x42/0xa060205a88:  [<6000aecf>] do_IRQ+0x2f/0x5060205aa8:  [<600154e4>] sig_handler_common+0x64/0xe060205b30:  [<6001b342>] __memcpy+0xe/0xac60205b50:  [<600ae3de>] sysfs_new_dirent+0xfe/0x12060205bd8:  [<600156aa>] sig_handler+0x1a/0x4060205be8:  [<60015983>] handle_signal+0x73/0xb060205c28:  [<60100140>] __restore_rt+0x0/0x1060205cd8:  [<6001b342>] __memcpy+0xe/0xac
Segmentation fault
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