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 v4:
- squashed patches #4 and #5 to fix a bisecting issue
v4:
https://lore.kernel.org/r/20260305-kflagstab-v4-0-6a76bf8b83c7@google.com
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>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
---
Siddharth Nayyar (7):
module: define ksym_flags enumeration to represent kernel symbol flags
module: add kflagstab section to vmlinux and modules
module: populate kflagstab in modpost
module: use kflagstab instead of *_gpl sections
module: deprecate usage of *_gpl sections in module loader
module: 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: 0138af2472dfdef0d56fc4697416eaa0ff2589bd
change-id: 20260305-kflagstab-51a08efed244
Best regards,
--
Siddharth Nayyar <sidnayyar@google.com>