Hi everyone, This patch series proposes a new, scalable mechanism to represent boolean flags for exported kernel symbols.
Problem Statement: The core architectural issue with kernel symbol flags is our reliance on splitting the main symbol table, ksymtab. To handle a single boolean property, such as GPL-only, all exported symbols are split across two separate tables: __ksymtab and __ksymtab_gpl. This design forces the module loader to perform a separate search on each of these tables for every symbol it needs, for vmlinux and for all previously loaded modules. This approach is fundamentally not scalable. If we were to introduce a second flag, we would need four distinct symbol tables. For n boolean flags, this model requires an exponential growth to 2^n tables, dramatically increasing complexity. Another consequence of this fragmentation is degraded performance. For example, a binary search on the symbol table of vmlinux, that would take only 14 comparison steps (assuming ~2^14 or 16K symbols) in a unified table, can require up to 26 steps when spread across two tables (assuming both tables have ~2^13 symbols). This performance penalty worsens as more flags are added. Proposed Solution: This series introduces a __kflagstab section to store symbol flags in a dedicated data structure, similar to how CRCs are handled in the __kcrctab. The flags for a given symbol in __kflagstab will be located at the same index as the symbol's entry in __ksymtab and its CRC in __kcrctab. This design decouples the flags from the symbol table itself, allowing us to maintain a single, sorted __ksymtab. As a result, the symbol search remains an efficient, single lookup, regardless of the number of flags we add in the future. The motivation for this change comes from the Android kernel, which uses an additional symbol flag to restrict the use of certain exported symbols by unsigned modules, thereby enhancing kernel security. This __kflagstab can be implemented as a bitmap to efficiently manage which symbols are available for general use versus those restricted to signed modules only. Patch Series Overview: * Patch 1-8: Introduce the __kflagstab, migrate the existing GPL-only flag to this new mechanism, and clean up the old __ksymtab_gpl infrastructure. * Patch 9-10: Add a "symbol import protection" flag, which disallows unsigned modules from importing symbols marked with this flag. This is an RFC, and I am seeking feedback on the overall approach and implementation before moving forward. Thanks, Siddharth Nayyar Siddharth Nayyar (10): define kernel symbol flags linker: add kflagstab section to vmlinux and modules modpost: create entries for kflagstab module loader: use kflagstab instead of *_gpl sections modpost: put all exported symbols in ksymtab section module loader: remove references of *_gpl sections linker: remove *_gpl sections from vmlinux and modules remove references to *_gpl sections in documentation modpost: add symbol import protection flag to kflagstab module loader: enforce symbol import protection Documentation/kbuild/modules.rst | 6 +- include/asm-generic/vmlinux.lds.h | 21 +++---- include/linux/export-internal.h | 28 ++++++--- include/linux/module.h | 4 +- include/linux/module_symbol.h | 6 ++ kernel/module/internal.h | 5 +- kernel/module/main.c | 101 ++++++++++++++---------------- scripts/mod/modpost.c | 27 ++++++-- scripts/module.lds.S | 3 +- 9 files changed, 107 insertions(+), 94 deletions(-) -- 2.51.0.338.gd7d06c2dae-goog
