module: define ksym_flags enumeration to represent kernel symbol flags

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.

To address this, symbol flags is an enumeration used to represent flags
as a bitset, for example a flag to tell if a symbol is GPL only.

The said bitset is introduced in subsequent patches and will contain
values of kernel symbol flags. These bitset will then be used to infer
flag values rather than fragmenting ksymtab for separating symbols with
different flag values, thereby eliminating the need to fragment the
ksymtab.

Link: https://lore.kernel.org/r/20260326-kflagstab-v5-0-fa0796fe88d9@google.com
Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
[Sami: Updated the commit message to explain the use case for the series.]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

diff --git a/include/linux/module_symbol.h b/include/linux/module_symbol.h
index 77c9895b9ddb19494bd87a34802102f476745578..574609aced991fc12869056c09f72ffa1eda2176 100644 (file)
--- a/include/linux/module_symbol.h
+++ b/include/linux/module_symbol.h
@@ -2,6 +2,11 @@
 #ifndef _LINUX_MODULE_SYMBOL_H
 #define _LINUX_MODULE_SYMBOL_H
 
+/* Kernel symbol flags bitset. */
+enum ksym_flags {
+       KSYM_FLAG_GPL_ONLY      = 1 << 0,
+};
+
 /* This ignores the intensely annoying "mapping symbols" found in ELF files. */
 static inline bool is_mapping_symbol(const char *str)
 {