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Linux kernel support for the 32-bit ARM architecture was contributed in the late 90s, when there was little corporate involvement in Linux development, and most contributors were students or hobbyists, tinkering with development boards, often without much in the way of documentation.
Now 20+ years later, 32-bit ARM’s maintainer has downgraded its support level to ‘odd fixes,’ while remaining active as a kernel contributor. This is a common pattern for aging and obsolete architectures: corporate funding for Linux kernel development has tremendously increased the pace of development, but only for architectures with a high return on investment. As a result, the 32-bit ARM port of Linux is essentially in maintenance-only mode, and lacks core Linux advancements such as THREAD_INFO_IN_TASK or VMAP_STACK, which protect against stack overflow attacks.
The lack of developer attention does not imply that the 32-bit ARM port has ceased to make economic sense, though. Instead, it has evolved from being one of the spearheads of Linux innovation to a stable and mature platform, and while funding its upstream development may not make sense in the long term, deploying 32-bit ARM into the field today most certainly still makes economic sense when margins are razor thin and BOM costs need to be kept to an absolute minimum. This is why 32-bit ARM is still widely used in embedded systems like set-top boxes and wireless routers.
Running 32-bit Linux on 64-bit ARM systems
Ironically, at these low price points, the DRAM is actually the dominant component in terms of BOM cost, and many of these 32-bit ARM systems incorporate a cheap ARMv8 SoC that happens to be capable of running in 64-bit mode as well. The reason for running 32-bit applications nonetheless is that these generally use less of the expensive DRAM, and can be
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