Add toolchain requirements blog post

Author: nikic

posts/2022-08-01-Increasing-glibc-kernel-requirements.md

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+---
+layout: post
+title: "Increasing the glibc and kernel requirements"
+author: Nikita Popov
+---
+
+The minimum requirements for Rust toolchains targeting Linux will [increase][PR] with the
+Rust 1.64.0 release (slated for September 22nd, 2022). The new minimum requirements are:
+
+* glibc >= 2.17 (previously glibc >= 2.11)
+* kernel >= 3.2 (previously kernel >= 2.6.32)
+
+These requirements apply both to running the Rust compiler itself (and other Rust tooling like
+Cargo or Rustup), and to running binaries produced by Rust, if they use the libstd.
+
+If you are not targeting an old long-term-support distribution, or embedded hardware running
+an old Linux version, this change is unlikely to affect you. Otherwise, read on!
+
+# Affected targets
+
+In principle, the new kernel requirements affect all `*-linux-*` targets, while the glibc
+requirements affect all `*-linux-gnu*` targets. In practice, many targets were already requiring
+newer kernel or glibc versions. The requirements for such targets do not change.
+
+Among targets for which a Rust host toolchain is distributed, the following *are* affected:
+
+* `i686-unknown-linux-gnu` (Tier 1)
+* `x86_64-unknown-linux-gnu` (Tier 1)
+* `x86_64-unknown-linux-musl` (Tier 2 with host tools)
+* `powerpc-unknown-linux-gnu` (Tier 2 with host tools)
+* `powerpc64-unknown-linux-gnu` (Tier 2 with host tools)
+* `s390x-unknown-linux-gnu` (Tier 2 with host tools)
+
+The following are *not* affected, because they already had higher glibc/kernel requirements:
+
+* `aarch64-unknown-linux-gnu` (Tier 1)
+* `aarch64-unknown-linux-musl` (Tier 2 with host tools)
+* `arm-unknown-linux-gnueabi` (Tier 2 with host tools)
+* `arm-unknown-linux-gnueabihf` (Tier 2 with host tools)
+* `armv7-unknown-linux-gnueabihf` (Tier 2 with host tools)
+* `mips-unknown-linux-gnueabihf` (Tier 2 with host tools)
+* `powerpc64le-unknown-linux-gnueabihf` (Tier 2 with host tools)
+* `riscv64gc-unknown-linux-gnueabihf` (Tier 2 with host tools)
+
+For other tier 2 or tier 3 targets, for which no Rust toolchain is distributed, we do not
+accurately track minimum requirements, and they may or may not be affected by this change.
+`*-linux-musl*` targets are only affected by the kernel requirements, not the glibc requirements.
+Targets which only use libcore and not libstd are unaffected.
+
+# Affected systems
+
+The glibc and kernel versions used for the new baseline requirements are already close to a decade
+old. As such, this change should only affect users that either target old long-term-support Linux
+distributions, or embedded hardware running old versions of Linux.
+
+The following Linux distributions *are* still supported under the new requirements:
+
+* RHEL 7 (glibc 2.17, kernel 3.10)
+* SLES 12-SP5 (glibc 2.22, kernel 4.12.14)
+* Debian 8 (glibc 2.19, kernel 3.16.7)
+* Ubuntu 14.04 (glibc 2.19, kernel 3.13)
+
+The following distributions are *not* supported under the new requirements:
+
+* RHEL 6 (glibc 2.12, kernel 2.6.32)
+* SLES 11-SP4 (glibc 2.11.3, kernel 3.0.101)
+* Debian 6 (glibc 2.11, kernel 2.6.32), Debian 7 (glibc 3.12, kernel 3.2.41)
+* Ubuntu 12.04 (glibc 2.15, kernel 3.2)
+
+Out of the distributions in the second list, only RHEL 6 still has limited vendor support (ELS).
+
+# Why increase the requirements?
+
+We want Rust, and binaries produced by Rust, to be as widely usable as possible. At the same time,
+the Rust project only has limited resources to maintain compatibility with old environments.
+
+There are two parts to the toolchain requirements: The minimum requirements for running the Rust
+compiler on a host system, and the minimum requirements for cross-compiled binaries.
+
+The minimum requirements for host toolchains affect our build system. Rust CI produces binary
+artifacts for dozens of different targets. Creating binaries that support old glibc versions
+requires either building on an operating system with old glibc (for native builds) or using a
+buildroot with an old glibc version (for cross-compiled builds).
+
+At the same time, Rust relies on LLVM for optimization and code generation, which regularly
+increases its toolchain requirements. LLVM 16 will require GCC 7.1 or newer (and LLVM 15 supports
+GCC 5.1 in name only). Creating a build environment that has both a very old glibc and a recent
+compiler becomes increasingly hard over time. crosstool-ng (which we use for most cross-compilation
+needs) does not support targeting both glibc 2.11, and using a compiler that satisfies the new LLVM
+requirements.
+
+The requirements for cross-compiled binaries have a different motivation: They affect which kernel
+versions need to be supported by libstd. Increasing the kernel requirements allows libstd to use
+newer syscalls, without having to maintain and test compatibility with kernels that do not support
+them.
+
+The new baseline requirements were picked as the least common denominator among long-term-support
+distributions that still have active support. This is currently RHEL 7 with glibc 2.17 and
+kernel 3.10. The kernel requirement is picked as 3.2 instead, because this is the minimum
+requirement of glibc itself, and there is little relevant API difference between these versions.
+
+# What should I do?
+
+If you or your organization are affected by this change, there are a number of viable options
+depending on your situation:
+
+* Upgrade your target system, or raise the minimum requirements of your software, to satisfy the
+  new constraints.
+* If you are running the Rust compiler on an old host, consider cross-compiling from a newer host
+  instead.
+* If you are targeting an old glibc version, consider targeting musl instead.
+* If you are targeting an old kernel version and use libstd, you may be out of luck: In this case
+  you may have to either freeze your current Rust version, or maintain a fork of libstd that
+  supports older kernels.
+
+[PR]: https://github.com/rust-lang/rust/pull/95026