Re: Clang patch stacks for LTS kernels (v4.4 and v4.9) and status update

From: Matthias Kaehlcke
Date: Tue Apr 24 2018 - 19:06:57 EST

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On Tue, Apr 24, 2018 at 01:54:29PM +0200, Sedat Dilek wrote:
> Hi Matthias,
>
> a big thank you for giving all the informations!
>
> I used your mka/llvm/v4.14_ext Git tree...
>
> https://chromium.googlesource.com/chromiumos/third_party/kernel/+log/sandbox/mka/llvm/v4.14_ext
>
> ...and was able to compile with clang-6.0 from Debian/buster64 with...
>
> ...reverting the clang-3/clang-4 patches...
>
> user$ for p in 69e44656ae43 222b88977a00 cdfcf1e45537 26f14c9225a6
> 0385a18e9995 68dab143c9b4 ; do echo [ $p ] ; LC_ALL=C git revert
> --no-edit $p ; done
>
> 0001-Revert-CLANG3-core-clang-work-around-x86-regparm-int.patch
> 0002-Revert-CLANG4-futex-don-t-optimize-futex_detect_cmpx.patch
> 0003-Revert-CLANG4-Disable-lkdtm-when-ftrace-is-enabled.patch
> 0004-Revert-CLANG4-arm64-prefetch-Use-__builtin_arm_prefe.patch
> 0005-Revert-CLANG4-kbuild-Add-meabi-gnu-to-the-clang-para.patch
> 0006-Revert-CLANG4-crypto-arm64-aes-ce-Explicitly-pass-th.patch
>
> ...and needed two additional patches from upstream:
>
> 0007-kbuild-clang-remove-crufty-HOSTCFLAGS.patch <--- Label with
> BACKPORT (XXX: Backported; Required when using HOSTCC in make-line,
> see below)
> 0008-x86-xen-remove-the-use-of-VLAIS.patch <--- Label with UPSTREAM
> (XXX: cherry-picked)
>
> These 3 patches in your Git branch are in Linux v4.14.36...
>
> 9af5ddf981ed BACKPORT: kbuild: disable clang's default use of
> -fmerge-all-constants
> f24088a3842c BACKPORT: kbuild: Set KBUILD_CFLAGS before incl. arch Makefile
> d4dfe384346d UPSTREAM: kbuild: fix linker feature test macros when
> cross compiling with Clang
>
> ...and can be dropped.
> I do not know your work-flow/policy: Maybe you want to keep your
> patch-stack against vanilla Linux v4.14 - without stables from
> linux-stable-4.14.y.

Yes, these patch stacks are based on vanilla Linux v4.14, one reason
is that it isn't a moving target. Also I think it makes it easier for
folks not merging LTS (though they probably should!) to locate all
patches, and it's fairly trivial to skip the (currently) few patches
not needed with the -stable tree.

> For easy switching "mycompiler" I use a wrapper-script:
>
> root# cat /usr/bin/mycompiler
> #!/bin/bash
>
> clang-6.0 "$@"
> - EOF -
>
> user$ cp -v /boot/config-4.14.35-1-iniza-amd64 .config
> user$ MAKE="make V=1" ; COMPILER="mycompiler" ;
> MAKE_OPTS="CC=$COMPILER HOSTCC=$COMPILER"
> user$ yes "" | $MAKE $MAKE_OPTS oldconfig && $MAKE $MAKE_OPTS
> silentoldconfig < /dev/null
>
> The diffconfig to my current kernel-config-4.14 looks like this...
>
> user$ ./scripts/diffconfig /boot/config-4.14.35-1-iniza-amd64 .config
> -ARCH_HAS_REFCOUNT y
> -BPF_JIT_ALWAYS_ON n
> -EXOFS_DEBUG n
> -EXOFS_FS m
> -GENERIC_CPU_VULNERABILITIES y
> -KASAN n
> -ORE m
> -PAGE_TABLE_ISOLATION y
> -RETPOLINE y
> -UNWINDER_FRAME_POINTER n
> -UNWINDER_GUESS n
> -UNWINDER_ORC y
> +FRAME_POINTER y
> +FRAME_POINTER_UNWINDER y
> +GUESS_UNWINDER n
> +HAVE_ARCH_KMEMCHECK y
> +HAVE_RELIABLE_STACKTRACE y
> +ORC_UNWINDER n
>
> Unfortunately, I cannot boot into the generated kernel on bare metal.
>
> Checking with QEMU (version: 2.12~rc3) and catching earlyprintk, I see this...
>
> user$ echo $KPATH
> $HOME/src/linux-kernel/important-files
>
> user$ ls -al
> insgesamt 344916
> drwxr-xr-x 2 sdi sdi 4096 Apr 24 13:15 .
> drwxr-xr-x 20 sdi sdi 4096 Apr 24 13:13 ..
> -rw-r--r-- 1 sdi sdi 4528416 Apr 24 12:42 bzImage
> lrwxrwxrwx 1 sdi sdi 35 Apr 24 13:15 initrd.img ->
> initrd.img-4.14.0-1-iniza-llvmlinux
> -rw-r--r-- 1 sdi sdi 25572955 Apr 24 13:08
> initrd.img-4.14.0-1-iniza-llvmlinux
> -rw-r--r-- 1 sdi sdi 2887195 Apr 24 12:42 System.map
> -rwxr-xr-x 1 sdi sdi 326116744 Apr 24 12:42 vmlinux
>
> user$ sudo qemu-system-x86_64 -enable-kvm -M pc -kernel $KPATH/bzImage
> -initrd $KPATH/initrd.img -m 512 -net none -serial stdio -append
> "root=/dev/ram0 console=ttyS0 hung_task_panic=1
> earlyprintk=ttyS0,115200"
>
> Probing EDD (edd=off to disable)... ok
> [ 0.000000] Linux version 4.14.0-1-iniza-llvmlinux
> (sedat.dilek@xxxxxxxxx@iniza) (clang version 6.0.0-1
> (tags/RELEASE_600/final)) #1 SMP Tue Apr 24 12:42:21 CEST 2018
> [ 0.000000] Command line: root=/dev/ram0 console=ttyS0
> hung_task_panic=1 earlyprintk=ttyS0,115200
> [ 0.000000] x86/fpu: x87 FPU will use FXSAVE
> [ 0.000000] e820: BIOS-provided physical RAM map:
> [ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
> [ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
> [ 0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
> [ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000001ffdffff] usable
> [ 0.000000] BIOS-e820: [mem 0x000000001ffe0000-0x000000001fffffff] reserved
> [ 0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
> [ 0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
> [ 0.000000] bootconsole [earlyser0] enabled
> [ 0.000000] NX (Execute Disable) protection: active
> [ 0.000000] random: fast init done
> [ 0.000000] SMBIOS 2.8 present.
> [ 0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.11.1-1 04/01/2014
> [ 0.000000] Hypervisor detected: KVM
> [ 0.000000] tsc: Fast TSC calibration using PIT
> [ 0.000000] e820: last_pfn = 0x1ffe0 max_arch_pfn = 0x400000000
> [ 0.000000] x86/PAT: PAT not supported by CPU.
> [ 0.000000] x86/PAT: Configuration [0-7]: WB WT UC- UC WB WT UC- UC
> Memory KASLR using RDTSC...
> [ 0.000000] found SMP MP-table at [mem 0x000f5d60-0x000f5d6f]
> mapped at [ffffffffff000d60]
> [ 0.000000] RAMDISK: [mem 0x1e77c000-0x1ffdffff]
> [ 0.000000] ACPI: Early table checksum verification disabled
> [ 0.000000] ACPI: RSDP 0x00000000000F5B90 000014 (v00 BOCHS )
> [ 0.000000] ACPI: RSDT 0x000000001FFE157C 000030 (v01 BOCHS
> BXPCRSDT 00000001 BXPC 00000001)
> [ 0.000000] ACPI: FACP 0x000000001FFE1458 000074 (v01 BOCHS
> BXPCFACP 00000001 BXPC 00000001)
> [ 0.000000] ACPI: DSDT 0x000000001FFE0040 001418 (v01 BOCHS
> BXPCDSDT 00000001 BXPC 00000001)
> [ 0.000000] ACPI: FACS 0x000000001FFE0000 000040
> [ 0.000000] ACPI: APIC 0x000000001FFE14CC 000078 (v01 BOCHS
> BXPCAPIC 00000001 BXPC 00000001)
> [ 0.000000] ACPI: HPET 0x000000001FFE1544 000038 (v01 BOCHS
> BXPCHPET 00000001 BXPC 00000001)
> [ 0.000000] No NUMA configuration found
> [ 0.000000] Faking a node at [mem 0x0000000000000000-0x000000001ffdffff]
> [ 0.000000] NODE_DATA(0) allocated [mem 0x1e777000-0x1e77bfff]
> [ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
> [ 0.000000] kvm-clock: cpu 0, msr 0:1e76f001, primary cpu clock
> [ 0.000000] kvm-clock: using sched offset of 528742140 cycles
> [ 0.000000] clocksource: kvm-clock: mask: 0xffffffffffffffff
> max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
> [ 0.000000] Zone ranges:
> [ 0.000000] DMA [mem 0x0000000000001000-0x0000000000ffffff]
> [ 0.000000] DMA32 [mem 0x0000000001000000-0x000000001ffdffff]
> [ 0.000000] Normal empty
> [ 0.000000] Device empty
> [ 0.000000] Movable zone start for each node
> [ 0.000000] Early memory node ranges
> [ 0.000000] node 0: [mem 0x0000000000001000-0x000000000009efff]
> [ 0.000000] node 0: [mem 0x0000000000100000-0x000000001ffdffff]
> [ 0.000000] Initmem setup node 0 [mem 0x0000000000001000-0x000000001ffdffff]
> [ 0.000000] ACPI: PM-Timer IO Port: 0x608
> [ 0.000000] ACPI: LAPIC_NMI (acpi_id[0xff] dfl dfl lint[0x1])
> [ 0.000000] IOAPIC[0]: apic_id 0, version 17, address 0xfec00000, GSI 0-23
> [ 0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 0 global_irq 2 dfl dfl)
> [ 0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 5 global_irq 5 high level)
> [ 0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 9 high level)
> [ 0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 10 global_irq 10 high level)
> [ 0.000000] ACPI: INT_SRC_OVR (bus 0 bus_irq 11 global_irq 11 high level)
> [ 0.000000] Using ACPI (MADT) for SMP configuration information
> [ 0.000000] ACPI: HPET id: 0x8086a201 base: 0xfed00000
> [ 0.000000] smpboot: Allowing 1 CPUs, 0 hotplug CPUs
> [ 0.000000] PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
> [ 0.000000] PM: Registered nosave memory: [mem 0x0009f000-0x0009ffff]
> [ 0.000000] PM: Registered nosave memory: [mem 0x000a0000-0x000effff]
> [ 0.000000] PM: Registered nosave memory: [mem 0x000f0000-0x000fffff]
> [ 0.000000] e820: [mem 0x20000000-0xfeffbfff] available for PCI devices
> [ 0.000000] Booting paravirtualized kernel on KVM
> [ 0.000000] clocksource: refined-jiffies: mask: 0xffffffff
> max_cycles: 0xffffffff, max_idle_ns: 7645519600211568 ns
> [ 0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512
> nr_cpu_ids:1 nr_node_ids:1
> [ 0.000000] percpu: Embedded 37 pages/cpu @ffff96dd9e400000 s114392
> r8192 d28968 u2097152
> [ 0.000000] KVM setup async PF for cpu 0
> [ 0.000000] kvm-stealtime: cpu 0, msr 1e40d900
> [ 0.000000] Built 1 zonelists, mobility grouping on. Total pages: 128872
> [ 0.000000] Policy zone: DMA32
> [ 0.000000] Kernel command line: root=/dev/ram0 console=ttyS0
> hung_task_panic=1 earlyprintk=ttyS0,115200
> [ 0.000000] PID hash table entries: 2048 (order: 2, 16384 bytes)
> [ 0.000000] Memory: 474480K/523768K available (7639K kernel code,
> 1005K rwdata, 2936K rodata, 1636K init, 688K bss, 49288K reserved, 0K
> cma-reserved)
> [ 0.000000] ftrace: allocating 27586 entries in 108 pages
> [ 0.004000] Hierarchical RCU implementation.
> [ 0.004000] RCU restricting CPUs from NR_CPUS=512 to nr_cpu_ids=1.
> [ 0.004000] RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=1
> [ 0.004000] NR_IRQS: 33024, nr_irqs: 256, preallocated irqs: 16
> [ 0.004000] Console: colour VGA+ 80x25
> [ 0.004000] console [ttyS0] enabled
> [ 0.004000] console [ttyS0] enabled
> [ 0.004000] bootconsole [earlyser0] disabled
> [ 0.004000] bootconsole [earlyser0] disabled
> [ 0.004000] clocksource: hpet: mask: 0xffffffff max_cycles:
> 0xffffffff, max_idle_ns: 19112604467 ns
> [ 0.004000] general protection fault: 0000 [#1] SMP
> [ 0.004000] Modules linked in:
> [ 0.004000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
> 4.14.0-1-iniza-llvmlinux #1
> [ 0.004000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.11.1-1 04/01/2014
> [ 0.004000] task: ffffffffaa610480 task.stack: ffffffffaa600000
> [ 0.004000] RIP: 0010:irq_work_tick+0x9d/0x110
> [ 0.004000] RSP: 0000:ffff96dd9e403e58 EFLAGS: 00010046
> [ 0.004000] RAX: 0000000000000082 RBX: ffff96dd9e411d80 RCX: adecc9cc04e2ca00
> [ 0.004000] RDX: 000000000001ba00 RSI: fffffffffffffed4 RDI: ffff96dd9e41ba38
> [ 0.004000] RBP: ffff96dd9e403e78 R08: 0000000000000000 R09: 0000000000000018
> [ 0.004000] R10: 0000000000000000 R11: 0000000000000018 R12: ffffffffaa61bd40
> [ 0.004000] R13: 0000000000000000 R14: ffffffffaa610480 R15: 0000000000000000
> [ 0.004000] FS: 0000000000000000(0000) GS:ffff96dd9e400000(0000)
> knlGS:0000000000000000
> [ 0.004000] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 0.004000] CR2: 00000000ffffffff CR3: 0000000008a09000 CR4: 00000000000006b0
> [ 0.004000] Call Trace:
> [ 0.004000] <IRQ>
> [ 0.004000] update_process_times+0x6e/0xa0
> [ 0.004000] tick_periodic+0x78/0x90
> [ 0.004000] tick_handle_periodic+0x26/0x80
> [ 0.004000] timer_interrupt+0x13/0x20
> [ 0.004000] __handle_irq_event_percpu+0x106/0x230
> [ 0.004000] handle_irq_event+0x5a/0xc0
> [ 0.004000] handle_level_irq+0x11a/0x190
> [ 0.004000] handle_irq+0x1f/0x30
> [ 0.004000] do_IRQ+0x4b/0xd0
> [ 0.004000] common_interrupt+0x93/0x93
> [ 0.004000] </IRQ>
> [ 0.004000] RIP: 0010:native_restore_fl+0x12/0x20
> [ 0.004000] RSP: 0000:ffffffffaa603e10 EFLAGS: 00000286 ORIG_RAX:
> ffffffffffffffcf
> [ 0.004000] RAX: 0000000000000001 RBX: ffff96dd9d0c0608 RCX: 0000000000000000
> [ 0.004000] RDX: ffff96dd9d0c0400 RSI: 0000000000000286 RDI: 0000000000000286
> [ 0.004000] RBP: ffffffffaa603e18 R08: 0000000000000001 R09: 000000000000003f
> [ 0.004000] R10: 0000000000000286 R11: 0000000000000007 R12: ffff96dd9d0c0514
> [ 0.004000] R13: ffff96dd9d0c04e0 R14: ffffffffaa61bd40 R15: ffff96dd9d0c0400
> [ 0.004000] _raw_spin_unlock_irqrestore+0x1a/0x20
> [ 0.004000] __setup_irq+0x5d9/0x780
> [ 0.004000] setup_irq+0x5c/0x90
> [ 0.004000] hpet_time_init+0x32/0x40
> [ 0.004000] x86_late_time_init+0x10/0x20
> [ 0.004000] start_kernel+0x45d/0x580
> [ 0.004000] x86_64_start_kernel+0x30f/0x320
> [ 0.004000] secondary_startup_64+0xa5/0xa5
> [ 0.004000] Code: f0 f0 4c 0f b1 7b f8 4c 89 e3 4d 85 e4 75 cf 48
> c7 c1 88 5b 01 00 65 48 03 0c 25 d8 a1 00 00 ff 14 25 10 08 62 aa f6
> c4 02 75 4d <48> 83 39 00 74 3e 31 db 48 87 19 48 85 db 74 34 0f 1f 00
> 48 8d
> [ 0.004000] RIP: irq_work_tick+0x9d/0x110 RSP: ffff96dd9e403e58
> [ 0.004000] ---[ end trace 8c7851007fbc6b6f ]---
> [ 0.004000] Kernel panic - not syncing: Fatal exception in interrupt
> [ 0.004000] ---[ end Kernel panic - not syncing: Fatal exception in interrupt
>
> Is this saying someting to you?

It doesn't ring a bell, but I can repro it (with different offsets):

[ 0.004000] general protection fault: 0000 [#1] SMP
[ 0.004000] Modules linked in:
[ 0.004000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.0+ #10
[ 0.004000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 0.004000] task: ffffffffbb410480 task.stack: ffffffffbb400000
[ 0.004000] RIP: 0010:irq_work_tick+0xaf/0x120
[ 0.004000] RSP: 0000:ffff96fb5fc03e58 EFLAGS: 00010046
[ 0.004000] RAX: 0000000000000082 RBX: ffff96fb5fc11d80 RCX: 47e992c1bc778d00
[ 0.004000] RDX: 000000000001ba00 RSI: fffffffffffffed4 RDI: ffff96fb5fc1ba38
[ 0.004000] RBP: ffff96fb5fc03e78 R08: 0000000000000000 R09: 0000000000000018
[ 0.004000] R10: 0000000000000000 R11: 0000000000000018 R12: ffffffffbb41bd40
[ 0.004000] R13: 0000000000000000 R14: ffffffffbb410480 R15: 0000000000000000
[ 0.004000] FS: 0000000000000000(0000) GS:ffff96fb5fc00000(0000) knlGS:0000000000000000
[ 0.004000] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 0.004000] CR2: 00000000ffffffff CR3: 000000001ec09000 CR4: 00000000000006b0
[ 0.004000] Call Trace:
[ 0.004000] <IRQ>
[ 0.004000] update_process_times+0x6e/0xa0
[ 0.004000] tick_periodic+0x78/0x90
[ 0.004000] tick_handle_periodic+0x26/0x80
[ 0.004000] timer_interrupt+0x13/0x20
[ 0.004000] __handle_irq_event_percpu+0x106/0x230
[ 0.004000] handle_irq_event+0x5a/0xc0
[ 0.004000] handle_level_irq+0x11a/0x190
[ 0.004000] handle_irq+0x1f/0x30
[ 0.004000] do_IRQ+0x4b/0xd0
[ 0.004000] common_interrupt+0x93/0x93
[ 0.004000] </IRQ>
[ 0.004000] RIP: 0010:native_restore_fl+0xd/0x20
[ 0.004000] RSP: 0000:ffffffffbb403e08 EFLAGS: 00000282 ORIG_RAX: ffffffffffffffcf
[ 0.004000] RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000000
[ 0.004000] RDX: ffff96fb5f01b800 RSI: 0000000000000282 RDI: 0000000000000282
[ 0.004000] RBP: ffffffffbb403e10 R08: 0000000000000001 R09: 000000000000003f
[ 0.004000] R10: ffff96fb5f01b800 R11: 0000000000000007 R12: ffff96fb5f01ba08
[ 0.004000] R13: ffff96fb5f01b8e0 R14: ffffffffbb41bd40 R15: ffff96fb5f01b800
[ 0.004000] _raw_spin_unlock_irqrestore+0x1a/0x20
[ 0.004000] __setup_irq+0x610/0x7b0
[ 0.004000] setup_irq+0x5b/0x90
[ 0.004000] hpet_time_init+0x32/0x40
[ 0.004000] x86_late_time_init+0x10/0x20
[ 0.004000] start_kernel+0x460/0x580
[ 0.004000] x86_64_start_kernel+0x30f/0x320
[ 0.004000] secondary_startup_64+0xa5/0xa5
[ 0.004000] Code: 4c 0f b1 7b f8 4c 89 e3 4d 85 e4 75 cf 48 c7 c1 88 5b 01 00 65 48 03 0c 25 d8 a1 00 00 ff 14 25 10 08 42 bb a9 00 02 00 00 75 4b <48> 83 39 00 74 3c 31 db 48 87 19 48 85 db 74 32 90 48 8d 7b f8
[ 0.004000] RIP: irq_work_tick+0xaf/0x120 RSP: ffff96fb5fc03e58
[ 0.004000] ---[ end trace 08945838e05bf5b2 ]---
[ 0.004000] Kernel panic - not syncing: Fatal exception in interrupt
[ 0.004000] ---[ end Kernel panic - not syncing: Fatal exception in interrupt

The exception occurs at 'irq_work_tick+0xaf'

objdump -d -S --start-address=0x$(grep irq_work_tick System.map | sed -e
"s/ \+.*//") vmlinux | less

...
void irq_work_tick(void)
{
ffffffff81193200: 55 push %rbp
...
static inline struct llist_node *llist_del_all(struct llist_head *head)
{
return xchg(&head->first, NULL);
ffffffff8119324a: 48 87 19 xchg %rbx,(%rcx)
while (llnode != NULL) {
ffffffff8119324d: 48 85 db test %rbx,%rbx
ffffffff81193250: 74 3f je ffffffff81193291 <irq_work_tick+0x91>
ffffffff81193252: 0f 1f 40 00 nopl 0x0(%rax)
ffffffff81193256: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
ffffffff8119325d: 00 00 00
work = llist_entry(llnode, struct irq_work, llnode);
ffffffff81193260: 48 8d 7b f8 lea -0x8(%rbx),%rdi
flags = work->flags & ~IRQ_WORK_PENDING;
ffffffff81193264: 4c 8b 7b f8 mov -0x8(%rbx),%r15
return node->next;
ffffffff81193268: 4c 8b 23 mov (%rbx),%r12
ffffffff8119326b: 4d 89 fe mov %r15,%r14
ffffffff8119326e: 49 83 e6 fe and $0xfffffffffffffffe,%r14
xchg(&work->flags, flags);
ffffffff81193272: 4c 89 f0 mov %r14,%rax
ffffffff81193275: 48 87 43 f8 xchg %rax,-0x8(%rbx)
work->func(work);
ffffffff81193279: ff 53 08 callq *0x8(%rbx)
(void)cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
ffffffff8119327c: 49 83 e7 fc and $0xfffffffffffffffc,%r15
ffffffff81193280: 4c 89 f0 mov %r14,%rax
ffffffff81193283: f0 4c 0f b1 7b f8 lock cmpxchg %r15,-0x8(%rbx)
ffffffff81193289: 4c 89 e3 mov %r12,%rbx
while (llnode != NULL) {
ffffffff8119328c: 4d 85 e4 test %r12,%r12
ffffffff8119328f: 75 cf jne ffffffff81193260 <irq_work_tick+0x60>
irq_work_run_list(raised);
irq_work_run_list(this_cpu_ptr(&lazy_list));
ffffffff81193291: 48 c7 c1 88 5b 01 00 mov $0x15b88,%rcx
ffffffff81193298: 65 48 03 0c 25 d8 a1 add %gs:0xa1d8,%rcx
ffffffff8119329f: 00 00
ffffffff811932a1: ff 14 25 10 08 c2 81 callq *0xffffffff81c20810
BUG_ON(!irqs_disabled());
ffffffff811932a8: a9 00 02 00 00 test $0x200,%eax
ffffffff811932ad: 75 4b jne ffffffff811932fa <irq_work_tick+0xfa>
return ACCESS_ONCE(head->first) == NULL;
ffffffff811932af: 48 83 39 00 cmpq $0x0,(%rcx)

I'm no x86/assembly expert, anyway my interpretation is:

'irq_work_tick+0xaf' corresponds to address 0xffffffff811932af, so the
exception occurs during the execution of 'cmpq $0x0,(%rcx)' or
'ACCESS_ONCE(head->first) == NULL'. The instruction checks if the
memory location in RCX contains 0. Supposedly the register should
contain the address of 'head->first', however the value is
0x47e992c1bc778d00, which doesn't look like a valid address.

RCX is set shortly before:

ffffffff81193291: 48 c7 c1 88 5b 01 00 mov $0x15b88,%rcx
ffffffff81193298: 65 48 03 0c 25 d8 a1 add %gs:0xa1d8,%rcx

As stated in https://www.kernel.org/doc/Documentation/this_cpu_ops.txt
the GS segment register is used by the kernel for per-cpu
variables. And effectively System.map tells us the offset 0x15b88
corresponds to the per-cpu variable lazy_list:

0000000000015b88 d lazy_list

The register dump shows GS with a value of 0xffff96fb5fc00000, which
looks reasonable.

But wait, right after setting RCX we do this:

ffffffff811932a1: ff 14 25 10 08 c2 81 callq *0xffffffff81c20810
BUG_ON(!irqs_disabled());

According to my (limited) understanding of x86 calling conventions RCX
is a caller-saved register, thus the caller should save it on the
stack to preserve its value across a function call.
(https://en.wikipedia.org/wiki/X86_calling_conventions)

The call to '*0xffffffff81c20810' appears to be related with
KVM/paravirtualization:

grep ffffffff81c20810 System.map
ffffffff81c20810 D pv_irq_ops

Since there is no offset it must be calling the first function in the
structure, which is 'safe_fl' and points to 'native_save_fl'
(https://elixir.bootlin.com/linux/v4.14.36/source/arch/x86/kernel/paravirt.c#L316)

objdump -d -S --start-address=0x$(grep native_save_fl System.map | sed
-e "s/ \+.*//") vmlinux | less

static inline unsigned long native_save_fl(void)
{
ffffffff81060240: 55 push %rbp
ffffffff81060241: 48 89 e5 mov %rsp,%rbp
ffffffff81060244: 48 83 ec 10 sub $0x10,%rsp
ffffffff81060248: 65 48 8b 04 25 28 00 mov %gs:0x28,%rax
ffffffff8106024f: 00 00
ffffffff81060251: 48 89 45 f8 mov %rax,-0x8(%rbp)
/*
* "=rm" is safe here, because "pop" adjusts the stack before
* it evaluates its effective address -- this is part of the
* documented behavior of the "pop" instruction.
*/
asm volatile("# __raw_save_flags\n\t"
ffffffff81060255: 9c pushfq
ffffffff81060256: 8f 45 f0 popq -0x10(%rbp)
"pushf ; pop %0"
: "=rm" (flags)
: /* no input */
: "memory");

return flags;
ffffffff81060259: 48 8b 45 f0 mov -0x10(%rbp),%rax
ffffffff8106025d: 65 48 8b 0c 25 28 00 mov %gs:0x28,%rcx
ffffffff81060264: 00 00
ffffffff81060266: 48 3b 4d f8 cmp -0x8(%rbp),%rcx
ffffffff8106026a: 75 06 jne ffffffff81060272 <native_save_fl+0x32>
ffffffff8106026c: 48 83 c4 10 add $0x10,%rsp
ffffffff81060270: 5d pop %rbp
ffffffff81060271: c3 retq
ffffffff81060272: e8 09 ec 01 00 callq ffffffff8107ee80 <__stack_chk_fail>
ffffffff81060277: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
ffffffff8106027e: 00 0

At 0xffffffff8106025d this clobbers RCX! I don't know why clang
doesn't save the value on the stack before calling native_save_fl(),
but that seems to be the problem.

Again, I'm not an expert in this area and ventured into territory
unknown to me, so please excuse if I got something totally wrong ...

For the record: a Chrome OS v4.14.35 kernel ('based' on the
sandbox/mka stack) built with clang still boots on an actual x86
device.

Matthias

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