Kernel: LWN's In-Depth Articles and AMD "Smart Trace Buffer" Feature Being Wired Up For Linux
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The intersection of modules, GKI, and rocket science [LWN.net]
One does not normally expect a lot of controversy around a patch series that makes changes to platform-specific configurations and drivers. The furor over some work on the Samsung Exynos platform may thus be surprising. When one looks into the discussion, things become more clear; it mostly has to do with disagreements over the best ways to get hardware vendors to cooperate with the kernel development community.
In mid-September, Will McVicker posted a brief series of changes for the Exynos configuration files; one week later, a larger, updated series followed. The purpose in both cases was to change a number of low-level system-on-chip (SoC) drivers to allow them to be built as loadable modules. That would seem like an uncontroversial change; it is normally expected that device drivers will be modular. But the situation is a little different for these low-level SoC drivers.
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Pulling slabs out of struct page [LWN.net]
For the time being, the effort to add the folio concept to the memory-management subsystem appears to be stalled, but appearances can be deceiving. The numerous folio discussions have produced a number of points of consensus, though; one of those is that far too much of the kernel has to work with page structures to get its job done. As an example of how a subsystem might be weaned off of struct page usage, Matthew Wilcox has split out the slab allocators in a 62-part patch set. The result may be a foreshadowing of changes to come in the memory-management subsystem.
The kernel maintains one page structure for every physical page of memory that it manages. On a typical system with a 4KB page size, that means managing millions of those structures. A page structure tells the kernel about the state of the page it refers to: how it is being used, how many references to it exist, its position in various queues, and more. The required information varies depending on how any given page is being used at the moment; to accommodate this, struct page is a complicated mess of structures and unions. The current definition of struct page makes for good pre-Halloween reading, but those who truly want a good scare may want to see what it looked like before Wilcox cleaned things up for 4.18.
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A rough start for ksmbd [LWN.net]
Among the many new features pulled into the mainline during the 5.15 merge window is the ksmbd network filesystem server. Ksmbd implements the SMB protocol (also known as CIFS, though that name has gone out of favor) that is heavily used in the Windows world. The creation of an in-kernel SMB server is a bit surprising, given that Linux has benefited greatly from the user-space Samba solution since shortly after the beginning. There are reasons for this move but, in the short term at least, they risk being overshadowed by a worrisome stream of security-related problems in ksmbd.
Why create an in-kernel SMB server at this point? In a sense, ksmbd is not meant to compete with Samba; indeed, it has been developed in cooperation with the Samba project. It is, however, meant to be a more performant and focused solution than Samba is; at this point, Samba includes a great deal of functionality beyond simple file serving. Ksmbd claims significant performance improvements on a wide range of benchmarks; the graphs on this page show a doubling of performance on some tests. An in-kernel server is an easier place to support variants like SMB Direct, which uses RDMA to transfer data between systems. By drawing more eyes to the code, merging into the mainline may also facilitate faster development in general. One other reason — which tends to be spoken rather more quietly — is that a new implementation can be licensed under GPLv2, while Samba is GPLv3.Ksmbd was first posted for review (as "cifsd") by Namjae Jeon in late March; the eighth revision came out just before the opening of the 5.15 merge window in late August. The last version received no review comments, but previous versions had clearly been looked at by a number of developers. Nobody objected when Steve French asked Linus Torvalds to pull ksmbd into the mainline on August 29.
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Scrutinizing bugs found by syzbot
The syzbot kernel-fuzzing system finds an enormous number of bugs, but, since many of them may seem to be of a relatively low severity, they have a lower priority when contending for the attention of developers. A talk at the recent Linux Security Summit North America reported on some research that dug further into the bugs that syzbot has found; the results are rather worrisome. Rather than a pile of difficult- or impossible-to-exploit bugs, there are numerous, more serious problems lurking within.
[...]
The first speaker on day one of the summit was Xiaochen Zou, a PhD student at the University of California, Riverside, who described a tool that he and his colleagues have been running on bugs that syzbot—which uses the syzkaller coverage-guided fuzzer—has reported in the Linux kernel. Over the last four years, syzbot has reported around 4000 bugs, of which 3000 have been fixed. There are eight categories that these bugs mainly fall into, but only some of those categories represent security bugs, he said. These are classic, severe kernel security holes, such as use after free, double free, and out-of-bounds writes; they are mostly found by the Kernel Address Sanitizer (KASAN) as part of syzkaller runs.
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AMD "Smart Trace Buffer" Feature Being Wired Up For Linux - Phoronix
AMD patches this month have begun preparing support for a new CPU feature called Smart Trace Buffer (STB).
Earlier this month the initial patch was posted for enabling AMD Smart Trace Buffer support on Linux as an extension to AMD's PMC kernel driver. This morning the STB enablement has already been revised in a second version of the patches.
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