5.8 Merge window, part 1

Architecture-specific

• Branch-target identification and shadow call stacks (both described in this article) have been added to the Arm64 architecture. Both are hardening technologies that, with luck, will make Arm64 systems more resistant to attack. The shadow call stack support is likely to spread to other architectures in the near future.

Core kernel

• The new faccessat2() system call adds the flags argument that POSIX has always said should be there. The current support for faccessat() on Linux systems depends on emulation of the flags argument by the C library; faccessat2() will allow a better implementation in the kernel.
• Memory control groups have a new knob, memory.swap.high, which can be used to slow down tasks that are using large amounts of swap space; see this commit for a bit more information.
• The io_uring subsystem now supports the tee() system call.
• It is now possible to pass a pidfd to the setns() system call; in that case, it is possible to specify multiple namespace types. The calling process will be moved to all of the applicable namespaces in an atomic manner.
• The "BPF iterator" mechanism, which facilitates the dumping of kernel data structures to user space, has been merged; this feature was covered in this article in April.
• There is a new ring buffer for communicating data from BPF programs. It is intended to resemble the perf ring buffer while allowing sharing of the buffer across multiple CPUs. See this documentation commit for more information.
• The padata mechanism now supports multi-threaded jobs with load balancing; see this documentation commit for details.
• The kernel's swappiness tuning knob, which sets the balance between reclaiming file-backed and anonymous pages, has traditionally been used to bias the system away from swapping anonymous pages. With fast I/O devices, though, swapping may be faster than filesystem access, so it may be useful to bias the system toward swapping. Now swappiness can take values up to 200 to push things in that direction; see this commit for details.

Filesystems and block I/O

• Low-level support for inline encryption has been added to the block layer. Inline encryption is a hardware feature that encrypts (and decrypts) data moving between a block storage device and the CPU using a key provided by the CPU. Some more information can be found in this commit.
• There is a new statx() flag (STATX_ATTR_DAX) that indicates that the file in question is being accessed directly via the DAX mechanism. There is also a documentation patch that attempts to specify just how filesystems will behave when DAX is in use. More DAX-related changes can be expected during this merge window.

Hardware support

• Graphics: Leadtek LTK050H3146W panels, Northwest Logic MIPI DSI host controllers, Chrontel CH7033 video encoders, Visionox RM69299 panels, and ASUS Z00T TM5P5 NT35596 panels.
• Hardware monitoring: Maxim MAX16601 voltage regulators, AMD RAPL MSR-based energy sensors, Gateworks System Controller analog-to-digital converters, and Baikal-T1 process, voltage, and temperature sensors.
• Interrupt control: Loongson3 HyperTransport interrupt vector controllers, Loongson PCH programmable interrupt controllers, and Loongson PCH MSI controllers.
• Media: Rockchip video decoders and OmniVision OV2740 sensors. The "atomisp" driver has also been resurrected in the staging tree and seen vast amounts of cleanup work.
• Miscellaneous: AMD SPI controllers, Maxim 77826 regulators, Arm CryptoCell true random number generators, Amlogic Meson SDHC host controllers, Freescale eSDHC ColdFire controllers, and Loongson PCI controllers,
• Networking: Broadcom BCM54140 PHYs, Qualcomm IPQ4019 MDIO interfaces, MediaTek STAR Ethernet MACs, Realtek 8723DE PCI wireless network adapters, and MediaTek MT7915E wireless interfaces.

Miscellaneous

• The new initrdmem= boot-time option specifies an initial disk image found in RAM; see this commit for more information.

Networking

• The bridge code now supports the media redundancy protocol, where a ring of Ethernet switches can be used to survive single-unit failures. See this commit for more information.
• The new "gate" action for the traffic-control subsystem allows specific packets to be passed into the system during specified time slots. This action is naturally undocumented, but some information can be found in this commit.
• Some network devices can perform testing of attached network cables; the kernel and ethtool utility now support that functionality when it is available.
• The multiprotocol label switching routing algorithm is now available for IPv6 as well as IPv4.
• RFC 8229, which describes encapsulation of key-exchange and IPSec packets, is now supported.

Security-related

• The CAP_PERFMON capability has been added; a process with this capability can do performance monitoring with the perf events subsystem.
• The new CAP_BPF capability covers some BPF operations that previously required CAP_SYS_ADMIN. In general, most BPF operations will also require either CAP_PERFMON (for tracing and such) or CAP_NET_ADMIN; this commit gives a terse overview of which operations require which capabilities.

Internal kernel changes

• The "pstore" mechanism, which stashes away system-state information in case of a panic, has gained a new back-end that stores data to a block device. See this commit for documentation.
• There is a new read-copy-update (RCU) variant called "RCU rude"; it delineates grace periods only at context switches. Those wondering about the name might see the comment in this commit, which reads: "It forces IPIs and context switches on all online CPUs, including idle ones, so use with caution".
• The RCU-tasks subsystem has a new "RCU tasks trace" variant suited to the needs of tracing and BPF programs; see this commit for details.
• "Local locks" have been brought over from the realtime preemption tree. These locks are intended to replace code that disables preemption and/or interrupts on a single processor. Advantages include a better realtime implementation and the ability to properly instrument locking; see this commit for more information.
• The API for managing file readahead has changed significantly; see this patch series for details.
• The kgdb kernel debugger is now able to work with the boot console, enabling debugging much earlier in the boot process; see this commit and this documentation patch for more information.
• There is a new buffer-allocation API intended to make the writing of XDP network drivers easier. Documentation is too much to hope for, but the API can be seen in this commit.

The 5.8 merge window can be expected to remain open until June 14; after that, the actual 5.8 release should happen in early August. Stay tuned; LWN will provide an update on the rest of this merge window after it closes.