Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst
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The purpose of supporting LSUI is to eliminate PAN toggling. CPUs that
support LSUI are unlikely to support a 32-bit runtime. Rather than
emulating the SWP instruction using LSUI instructions in order to remove
PAN toggling, simply disable SWP emulation.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: some tweaks to the in-code comment]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Since Armv9.6, FEAT_LSUI supplies the load/store instructions for
previleged level to access to access user memory without clearing
PSTATE.PAN bit.
Add Kconfig option entry for FEAT_LSUI.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Use the CAST instruction to swap the guest descriptor when FEAT_LSUI
is enabled, avoiding the need to clear the PAN bit.
FEAT_LSUI is introduced in Armv9.6, where FEAT_PAN is mandatory. However,
this assumption may not always hold:
- Some CPUs may advertise FEAT_LSUI but lack FEAT_PAN.
- Virtualization or ID register overrides may expose invalid feature
combinations.
Therefore, instead of disabling FEAT_LSUI when FEAT_PAN is absent, wrap
LSUI instructions with uaccess_ttbr0_enable()/disable() when
ARM64_SW_TTBR0_PAN is enabled.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Current futex atomic operations are implemented using LL/SC instructions
while temporarily clearing PSTATE.PAN and setting PSTATE.TCO (if
KASAN_HW_TAGS is enabled). With Armv9.6, FEAT_LSUI provides atomic
instructions for user memory access in the kernel without the need for
PSTATE bits toggling.
Use the FEAT_LSUI instructions to implement the futex atomic operations.
Note that some futex operations do not have a matching LSUI instruction,
(eor or word-sized cmpxchg). For such cases, use cas{al}t to implement
the operation.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: add comment on -EAGAIN in __lsui_futex_cmpxchg()]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Refactor the futex atomic operations using ll/sc instructions in
preparation for FEAT_LSUI support. In addition, use named operands for
the inline asm.
No functional change.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: remove unnecessary stringify.h include]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Add test coverage for FEAT_LSUI.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Expose the FEAT_LSUI ID register field to guests.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Since Armv9.6, FEAT_LSUI introduces atomic instructions that allow
privileged code to access user memory without clearing the PSTATE.PAN
bit. Add CPU feature detection for FEAT_LSUI.
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
[catalin.marinas@arm.com: Remove commit log references to SW_PAN]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Pull EFI fix from Ard Biesheuvel:
"Fix for the x86 EFI workaround keeping boot services code and data
regions reserved until after SetVirtualAddressMap() completes:
deferred struct page initialization may result in some of this memory
being lost permanently"
* tag 'efi-fixes-for-v7.0-2' of git://git.kernel.org/pub/scm/linux/kernel/git/efi/efi:
x86/efi: defer freeing of boot services memory
Pull i2c fix from Wolfram Sang:
"A revert for the i801 driver restoring old locking behaviour"
* tag 'i2c-for-7.0-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux:
i2c: i801: Revert "i2c: i801: replace acpi_lock with I2C bus lock"
efi_free_boot_services() frees memory occupied by EFI_BOOT_SERVICES_CODE
and EFI_BOOT_SERVICES_DATA using memblock_free_late().
There are two issue with that: memblock_free_late() should be used for
memory allocated with memblock_alloc() while the memory reserved with
memblock_reserve() should be freed with free_reserved_area().
More acutely, with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
efi_free_boot_services() is called before deferred initialization of the
memory map is complete.
Benjamin Herrenschmidt reports that this causes a leak of ~140MB of
RAM on EC2 t3a.nano instances which only have 512MB or RAM.
If the freed memory resides in the areas that memory map for them is
still uninitialized, they won't be actually freed because
memblock_free_late() calls memblock_free_pages() and the latter skips
uninitialized pages.
Using free_reserved_area() at this point is also problematic because
__free_page() accesses the buddy of the freed page and that again might
end up in uninitialized part of the memory map.
Delaying the entire efi_free_boot_services() could be problematic
because in addition to freeing boot services memory it updates
efi.memmap without any synchronization and that's undesirable late in
boot when there is concurrency.
More robust approach is to only defer freeing of the EFI boot services
memory.
Split efi_free_boot_services() in two. First efi_unmap_boot_services()
collects ranges that should be freed into an array then
efi_free_boot_services() later frees them after deferred init is complete.
Link: https://lore.kernel.org/all/ec2aaef14783869b3be6e3c253b2dcbf67dbc12a.camel@kernel.crashing.org
Fixes: 916f676f8dc0 ("x86, efi: Retain boot service code until after switching to virtual mode")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Pull x86 fixes from Ingo Molnar:
- Fix SEV guest boot failures in certain circumstances, due to
very early code relying on a BSS-zeroed variable that isn't
actually zeroed yet an may contain non-zero bootup values
Move the variable into the .data section go gain even earlier
zeroing
- Expose & allow the IBPB-on-Entry feature on SNP guests, which
was not properly exposed to guests due to initial implementational
caution
- Fix O= build failure when CONFIG_EFI_SBAT_FILE is using relative
file paths
- Fix the various SNC (Sub-NUMA Clustering) topology enumeration
bugs/artifacts (sched-domain build errors mostly).
SNC enumeration data got more complicated with Granite Rapids X
(GNR) and Clearwater Forest X (CWF), which exposed these bugs
and made their effects more serious
- Also use the now sane(r) SNC code to fix resctrl SNC detection bugs
- Work around a historic libgcc unwinder bug in the vdso32 sigreturn
code (again), which regressed during an overly aggressive recent
cleanup of DWARF annotations
* tag 'x86-urgent-2026-03-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
x86/entry/vdso32: Work around libgcc unwinder bug
x86/resctrl: Fix SNC detection
x86/topo: Fix SNC topology mess
x86/topo: Replace x86_has_numa_in_package
x86/topo: Add topology_num_nodes_per_package()
x86/numa: Store extra copy of numa_nodes_parsed
x86/boot: Handle relative CONFIG_EFI_SBAT_FILE file paths
x86/sev: Allow IBPB-on-Entry feature for SNP guests
x86/boot/sev: Move SEV decompressor variables into the .data section
This reverts commit f707d6b9e7c18f669adfdb443906d46cfbaaa0c1.
Under rare circumstances, multiple udev threads can collect i801 device
info on boot and walk i801_acpi_io_handler somewhat concurrently. The
first will note the area is reserved by acpi to prevent further touches.
This ultimately causes the area to be deregistered. The second will
enter i801_acpi_io_handler after the area is unregistered but before a
check can be made that the area is unregistered. i2c_lock_bus relies on
the now unregistered area containing lock_ops to lock the bus. The end
result is a kernel panic on boot with the following backtrace;
[ 14.971872] ioatdma 0000:09:00.2: enabling device (0100 -> 0102)
[ 14.971873] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 14.971880] #PF: supervisor read access in kernel mode
[ 14.971884] #PF: error_code(0x0000) - not-present page
[ 14.971887] PGD 0 P4D 0
[ 14.971894] Oops: 0000 [#1] PREEMPT SMP PTI
[ 14.971900] CPU: 5 PID: 956 Comm: systemd-udevd Not tainted 5.14.0-611.5.1.el9_7.x86_64 #1
[ 14.971905] Hardware name: XXXXXXXXXXXXXXXXXXXXXXX BIOS 1.20.10.SV91 01/30/2023
[ 14.971908] RIP: 0010:i801_acpi_io_handler+0x2d/0xb0 [i2c_i801]
[ 14.971929] Code: 00 00 49 8b 40 20 41 57 41 56 4d 8b b8 30 04 00 00 49 89 ce 41 55 41 89 d5 41 54 49 89 f4 be 02 00 00 00 55 4c 89 c5 53 89 fb <48> 8b 00 4c 89 c7 e8 18 61 54 e9 80 bd 80 04 00 00 00 75 09 4c 3b
[ 14.971933] RSP: 0018:ffffbaa841483838 EFLAGS: 00010282
[ 14.971938] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff9685e01ba568
[ 14.971941] RDX: 0000000000000008 RSI: 0000000000000002 RDI: 0000000000000000
[ 14.971944] RBP: ffff9685ca22f028 R08: ffff9685ca22f028 R09: ffff9685ca22f028
[ 14.971948] R10: 000000000000000b R11: 0000000000000580 R12: 0000000000000580
[ 14.971951] R13: 0000000000000008 R14: ffff9685e01ba568 R15: ffff9685c222f000
[ 14.971954] FS: 00007f8287c0ab40(0000) GS:ffff96a47f940000(0000) knlGS:0000000000000000
[ 14.971959] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 14.971963] CR2: 0000000000000000 CR3: 0000000168090001 CR4: 00000000003706f0
[ 14.971966] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 14.971968] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 14.971972] Call Trace:
[ 14.971977] <TASK>
[ 14.971981] ? show_trace_log_lvl+0x1c4/0x2df
[ 14.971994] ? show_trace_log_lvl+0x1c4/0x2df
[ 14.972003] ? acpi_ev_address_space_dispatch+0x16e/0x3c0
[ 14.972014] ? __die_body.cold+0x8/0xd
[ 14.972021] ? page_fault_oops+0x132/0x170
[ 14.972028] ? exc_page_fault+0x61/0x150
[ 14.972036] ? asm_exc_page_fault+0x22/0x30
[ 14.972045] ? i801_acpi_io_handler+0x2d/0xb0 [i2c_i801]
[ 14.972061] acpi_ev_address_space_dispatch+0x16e/0x3c0
[ 14.972069] ? __pfx_i801_acpi_io_handler+0x10/0x10 [i2c_i801]
[ 14.972085] acpi_ex_access_region+0x5b/0xd0
[ 14.972093] acpi_ex_field_datum_io+0x73/0x2e0
[ 14.972100] acpi_ex_read_data_from_field+0x8e/0x230
[ 14.972106] acpi_ex_resolve_node_to_value+0x23d/0x310
[ 14.972114] acpi_ds_evaluate_name_path+0xad/0x110
[ 14.972121] acpi_ds_exec_end_op+0x321/0x510
[ 14.972127] acpi_ps_parse_loop+0xf7/0x680
[ 14.972136] acpi_ps_parse_aml+0x17a/0x3d0
[ 14.972143] acpi_ps_execute_method+0x137/0x270
[ 14.972150] acpi_ns_evaluate+0x1f4/0x2e0
[ 14.972158] acpi_evaluate_object+0x134/0x2f0
[ 14.972164] acpi_evaluate_integer+0x50/0xe0
[ 14.972173] ? vsnprintf+0x24b/0x570
[ 14.972181] acpi_ac_get_state.part.0+0x23/0x70
[ 14.972189] get_ac_property+0x4e/0x60
[ 14.972195] power_supply_show_property+0x90/0x1f0
[ 14.972205] add_prop_uevent+0x29/0x90
[ 14.972213] power_supply_uevent+0x109/0x1d0
[ 14.972222] dev_uevent+0x10e/0x2f0
[ 14.972228] uevent_show+0x8e/0x100
[ 14.972236] dev_attr_show+0x19/0x40
[ 14.972246] sysfs_kf_seq_show+0x9b/0x100
[ 14.972253] seq_read_iter+0x120/0x4b0
[ 14.972262] ? selinux_file_permission+0x106/0x150
[ 14.972273] vfs_read+0x24f/0x3a0
[ 14.972284] ksys_read+0x5f/0xe0
[ 14.972291] do_syscall_64+0x5f/0xe0
...
The kernel panic is mitigated by setting limiting the count of udev
children to 1. Revert to using the acpi_lock to continue protecting
marking the area as owned by firmware without relying on a lock in
a potentially unmapped region of memory.
Fixes: f707d6b9e7c1 ("i2c: i801: replace acpi_lock with I2C bus lock")
Signed-off-by: Charles Haithcock <chaithco@redhat.com>
[wsa: added Fixes-tag and updated comment stating the importance of the lock]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Pull timer fix from Ingo Molnar:
"Make clock_adjtime() syscall timex validation slightly more permissive
for auxiliary clocks, to not reject syscalls based on the status field
that do not try to modify the status field.
This makes the ABI behavior in clock_adjtime() consistent with
CLOCK_REALTIME"
* tag 'timers-urgent-2026-03-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
timekeeping: Fix timex status validation for auxiliary clocks