[PATCH v4 0/8] scalable symbol flags with __kflagstab

[Siddharth Nayyar <sidnayyar@google.com>]

This patch series implements a mechanism for scalable exported symbol
flags using a separate section called __kflagstab. The series introduces
__kflagstab support, removes *_gpl sections in favor of a GPL flag,
simplifies symbol resolution during module loading.

The __kflagstab contains an 8-bit bitset which can represent up to 8
boolean flags per symbol exported in the __ksymtab. The patch series
also uses this bitset to store GPL-only flag values for kernel symbols,
thereby eliminating the need for *_gpl sections for representing GPL
only symbols.

Petr Pavlu ran a small test to get a better understanding of the
different section sizes resulting from this patch series.  He used
v6.17-rc6 together with the openSUSE x86_64 config [1], which is fairly
large. The resulting vmlinux.bin (no debuginfo) had an on-disk size of
58 MiB, and included 5937 + 6589 (GPL-only) exported symbols.

The following table summarizes his measurements and calculations
regarding the sizes of all sections related to exported symbols:

                      |  HAVE_ARCH_PREL32_RELOCATIONS  | !HAVE_ARCH_PREL32_RELOCATIONS
 Section              | Base [B] | Ext. [B] | Sep. [B] | Base [B] | Ext. [B] | Sep. [B]
----------------------------------------------------------------------------------------
 __ksymtab            |    71244 |   200416 |   150312 |   142488 |   400832 |   300624
 __ksymtab_gpl        |    79068 |       NA |       NA |   158136 |       NA |       NA
 __kcrctab            |    23748 |    50104 |    50104 |    23748 |    50104 |    50104
 __kcrctab_gpl        |    26356 |       NA |       NA |    26356 |       NA |       NA
 __ksymtab_strings    |   253628 |   253628 |   253628 |   253628 |   253628 |   253628
 __kflagstab          |       NA |       NA |    12526 |       NA |       NA |    12526
----------------------------------------------------------------------------------------
 Total                |   454044 |   504148 |   466570 |   604356 |   704564 |   616882
 Increase to base [%] |       NA |     11.0 |      2.8 |       NA |     16.6 |      2.1

The column "HAVE_ARCH_PREL32_RELOCATIONS -> Base" contains themeasured
numbers. The rest of the values are calculated. The "Ext." column
represents an alternative approach of extending __ksymtab to include a
bitset of symbol flags, and the "Sep." column represents the approach of
having a separate __kflagstab. With HAVE_ARCH_PREL32_RELOCATIONS, each
kernel_symbol is 12 B in size and is extended to 16 B. With
!HAVE_ARCH_PREL32_RELOCATIONS, it is 24 B, extended to 32 B. Note that
this does not include the metadata needed to relocate __ksymtab*, which
is freed after the initial processing.

The base export data in this case totals 0.43 MiB. About 50% is used for
storing the names of exported symbols.

Adding __kflagstab as a separate section has a negligible impact, as
expected. When extending __ksymtab (kernel_symbol) instead, the worst
case with !HAVE_ARCH_PREL32_RELOCATIONS increases the export data size
by 16.6%. Note that the larger increase in size for the latter approach
is due to 4-byte alignment of kernel_symbol data structure, instead of
1-byte alignment for the flags bitset in __kflagstab in the former
approach.

Based on the above, it was concluded that introducing __kflagstab makes
senses, as the added complexity is minimal over extending kernel_symbol,
and there is overall simplification of symbol finding logic in the
module loader.

Thank you Petr Pavlu for doing a section size analysis as well as Sami
Tolvanen, Petr Pavlu and Jonathan Corbet for their valuable feedback.

---
Changes from v3:
- made commit messages more descriptive

v3:
https://lore.kernel.org/20251103161954.1351784-1-sidnayyar@google.com/

Changes from v2:
- dropped symbol import protection to spin off into its own series

v2:
https://lore.kernel.org/20251013153918.2206045-1-sidnayyar@google.com/

Changes from v1:
- added a check to ensure __kflagstab is present
- added warnings for the obsolete *_gpl sections
- moved protected symbol check before ref_module() call
- moved protected symbol check failure warning to issue detection point

v1:
https://lore.kernel.org/20250829105418.3053274-1-sidnayyar@google.com/

[1] https://github.com/openSUSE/kernel-source/blob/307f149d9100a0e229eb94cbb997ae61187995c3/config/x86_64/default

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>

---
Siddharth Nayyar (8):
      define ksym_flags enumeration to represent kernel symbol flags
      linker: add kflagstab section to vmlinux and modules
      modpost: populate kflagstab
      module loader: use kflagstab instead of *_gpl sections
      modpost: remove fragmentation of ksymtab and kcrctab sections
      module loader: deprecate usage of *_gpl sections
      linker: remove *_gpl sections from vmlinux and modules
      documentation: remove references to *_gpl sections

 Documentation/kbuild/modules.rst  |  11 +++--
 include/asm-generic/vmlinux.lds.h |  21 +++-----
 include/linux/export-internal.h   |  28 +++++++----
 include/linux/module.h            |   4 +-
 include/linux/module_symbol.h     |   5 ++
 kernel/module/internal.h          |   4 +-
 kernel/module/main.c              | 101 ++++++++++++++++++--------------------
 scripts/mod/modpost.c             |  16 ++++--
 scripts/module.lds.S              |   3 +-
 9 files changed, 98 insertions(+), 95 deletions(-)
---
base-commit: c107785c7e8dbabd1c18301a1c362544b5786282
change-id: 20260305-kflagstab-51a08efed244

Best regards,
-- 
Siddharth Nayyar <sidnayyar@google.com>