module: add kflagstab section to vmlinux and modules

This patch 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.

This section will contain read-only data for values of kernel symbol
flags in the form of an 8-bit bitsets for each kernel symbol. Each bit
in the bitset represents a flag value defined by ksym_flags enumeration.

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 the measured
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.

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
sense, as the added complexity is minimal over extending kernel_symbol,
and there is overall simplification of symbol finding logic in the
module loader.

Signed-off-by: Siddharth Nayyar <sidnayyar@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
[Sami: Updated commit message to include details from the cover letter.]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 1e1580febe4b9a78d30bef72c7ea942c412833a3..d64a475c468ae51365cf53348758e770001bbf09 100644 (file)
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -536,6 +536,13 @@
                __stop___kcrctab_gpl = .;                               \
        }                                                               \
                                                                        \
+       /* Kernel symbol flags table */                                 \
+       __kflagstab       : AT(ADDR(__kflagstab) - LOAD_OFFSET) {       \
+               __start___kflagstab = .;                                \
+               KEEP(*(SORT(___kflagstab+*)))                           \
+               __stop___kflagstab = .;                                 \
+       }                                                               \
+                                                                       \
        /* Kernel symbol table: strings */                              \
         __ksymtab_strings : AT(ADDR(__ksymtab_strings) - LOAD_OFFSET) {        \
                *(__ksymtab_strings)                                    \

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index e1cab3cee3f7da71046dd46146316d0d72f81382..3ecfb3ea1cc8bee3644dca29e3e1d40f75ed581f 100644 (file)
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -23,6 +23,7 @@ SECTIONS {
        __ksymtab_gpl           0 : ALIGN(8) { *(SORT(___ksymtab_gpl+*)) }
        __kcrctab               0 : ALIGN(4) { *(SORT(___kcrctab+*)) }
        __kcrctab_gpl           0 : ALIGN(4) { *(SORT(___kcrctab_gpl+*)) }
+       __kflagstab             0 : ALIGN(1) { *(SORT(___kflagstab+*)) }
 
        .ctors                  0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) }
        .init_array             0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) }