CONFIG_TEST_KALLSYMS: module kallsyms find_symbol() test

General informations

The Linux kernel configuration item CONFIG_TEST_KALLSYMS:

• prompt: module kallsyms find_symbol() test
• type: tristate
• depends on: CONFIG_m
• defined in lib/Kconfig.debug
• found in Linux kernels: 6.13–6.14

Help text

This allows us to stress test find_symbol() through the kallsyms used to place symbols on the kernel ELF kallsyms and modules kallsyms where we place kernel symbols such as exported symbols.

We have four test modules:

A: has KALLSYSMS_NUMSYMS exported symbols B: uses one of A's symbols C: adds KALLSYMS_SCALE_FACTOR * KALLSYSMS_NUMSYMS exported D: adds 2 * the symbols than C

We stress test find_symbol() through two means:

1) Upon load of B it will trigger simplify_symbols() to look for the one symbol it uses from the module A with tons of symbols. This is an indirect way for us to have B call resolve_symbol_wait() upon module load. This will eventually call find_symbol() which will eventually try to find the symbols used with find_exported_symbol_in_section(). find_exported_symbol_in_section() uses bsearch() so a binary search for each symbol. Binary search will at worst be O(log(n)) so the larger TEST_MODULE_KALLSYSMS the worse the search.

2) The selftests should load C first, before B. Upon B's load towards the end right before we call module B's init routine we get complete_formation() called on the module. That will first check for duplicate symbols with the call to verify_exported_symbols(). That is when we'll force iteration on module C's insane symbol list. Since it has 10 * KALLSYMS_NUMSYMS it means we can first test just loading B without C. The amount of time it takes to load C Vs B can give us an idea of the impact growth of the symbol space and give us projection. Module A only uses one symbol from B so to allow this scaling in module C to be proportional, if it used more symbols then the first test would be doing more and increasing just the search space would be slightly different. The last module, module D will just increase the search space by twice the number of symbols in C so to allow for full projects.

tools/testing/selftests/module/find_symbol.sh

The current defaults will incur a build delay of about 7 minutes on an x86_64 with only 8 cores. Enable this only if you want to stress test find_symbol() with thousands of symbols. At the same time this is also useful to test building modules with thousands of symbols, and if BTF is enabled this also stress tests adding BTF information for each module. Currently enabling many more symbols will segfault the build system.

If unsure, say N.

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The data is retrived from:

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• Linux Kernel Driver DataBase (LKDDb)

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