Zack's Kernel News

Zack's Kernel News

Article from Issue 260/2022

Zack Brown reports on taking a header and the next Spectre vulnerability.

Taking a Header

In the Winter of 2020, Ingo Molnar decided that something simply had to be done to make everyone's life better. He reached into his ultimate sack of horrible things and pulled out the Linux kernel header hierarchy. This oozing nightmare consisted of all the header files in the kernel source tree, one depending upon the other in an endless glutinous web that could be neither untangled nor untied and that all kernel sub-projects simply glom onto, forming endless sticky layers upon which the fate of humanity truly does depend.

So Ingo untangled and untied it using determination and strange gifts. Then recently he submitted a patch, consisting of over 25 sub-trees, with over 2,200 individual commits, changing more than half of all source files in the entire kernel tree. He said, "As most kernel developers know, there's around ~10,000 main .h headers in the Linux kernel, in the include/ and arch/*/include/ hierarchies. Over the last 30+ years they have grown into a complicated & painful set of cross-dependencies we are affectionately calling 'Dependency Hell'."

He offered his patch to the world, calling it the Fast Kernel Headers project. According to his tests, it would cut kernel compile times down to as much as one fifth of what they had been. Incremental compile times – where files compiled earlier don't need to be recompiled – were even more drastically improved. The oozing web had become a delicate lace – or at least less hellish.


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