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As promised, for v5.19 I queued up quite a bit of work for modules, but
still with a pretty conservative eye. These changes have been soaking on
modules-next (and so linux-next) for quite some time, the code shift was
merged onto modules-next on March 22, and the last patch was queued on May
5th.
The following are the highlights of what bells and whistles we will get for
v5.19:
1) It was time to tidy up kernel/module.c and one way of starting with
that effort was to split it up into files. At my request Aaron Tomlin
spearheaded that effort with the goal to not introduce any
functional at all during that endeavour. The penalty for the split
is +1322 bytes total, +112 bytes in data, +1210 bytes in text while
bss is unchanged. One of the benefits of this other than helping
make the code easier to read and review is summoning more help on review
for changes with livepatching so kernel/module/livepatch.c is now
pegged as maintained by the live patching folks.
The before and after with just the move on a defconfig on x86-64:
$ size kernel/module.o
text data bss dec hex filename
38434 4540 104 43078 a846 kernel/module.o
$ size -t kernel/module/*.o
text data bss dec hex filename
4785 120 0 4905 1329 kernel/module/kallsyms.o
28577 4416 104 33097 8149 kernel/module/main.o
1158 8 0 1166 48e kernel/module/procfs.o
902 108 0 1010 3f2 kernel/module/strict_rwx.o
3390 0 0 3390 d3e kernel/module/sysfs.o
832 0 0 832 340 kernel/module/tree_lookup.o
39644 4652 104 44400 ad70 (TOTALS)
2) Aaron added module unload taint tracking (MODULE_UNLOAD_TAINT_TRACKING),
so to enable tracking unloaded modules which did taint the kernel.
3) Christophe Leroy added CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC
which lets architectures to request having modules data in vmalloc
area instead of module area. There are three reasons why an
architecture might want this:
a) On some architectures (like book3s/32) it is not possible to protect
against execution on a page basis. The exec stuff can be mapped by
different arch segment sizes (on book3s/32 that is 256M segments). By
default the module area is in an Exec segment while vmalloc area is in
a NoExec segment. Using vmalloc lets you muck with module data as
NoExec on those architectures whereas before you could not.
b) By pushing more module data to vmalloc you also increase the
probability of module text to remain within a closer distance
from kernel core text and this reduces trampolines, this has been
reported on arm first and powerpc folks are following that lead.
c) Free'ing module_alloc() (Exec by default) area leaves this
exposed as Exec by default, some architectures have some
security enhancements to set this as NoExec on free, and splitting
module data with text let's future generic special allocators
be added to the kernel without having developers try to grok
the tribal knowledge per arch. Work like Rick Edgecombe's
permission vmalloc interface [0] becomes easier to address over
time.
[0] https://lore.kernel.org/lkml/20201120202426.18009-1-rick.p.edgecombe@intel.com/#r
4) Masahiro Yamada's symbol search enhancements
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Merge tag 'modules-5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux
Pull modules updates from Luis Chamberlain:
- It was time to tidy up kernel/module.c and one way of starting with
that effort was to split it up into files. At my request Aaron Tomlin
spearheaded that effort with the goal to not introduce any functional
at all during that endeavour. The penalty for the split is +1322
bytes total, +112 bytes in data, +1210 bytes in text while bss is
unchanged. One of the benefits of this other than helping make the
code easier to read and review is summoning more help on review for
changes with livepatching so kernel/module/livepatch.c is now pegged
as maintained by the live patching folks.
The before and after with just the move on a defconfig on x86-64:
$ size kernel/module.o
text data bss dec hex filename
38434 4540 104 43078 a846 kernel/module.o
$ size -t kernel/module/*.o
text data bss dec hex filename
4785 120 0 4905 1329 kernel/module/kallsyms.o
28577 4416 104 33097 8149 kernel/module/main.o
1158 8 0 1166 48e kernel/module/procfs.o
902 108 0 1010 3f2 kernel/module/strict_rwx.o
3390 0 0 3390 d3e kernel/module/sysfs.o
832 0 0 832 340 kernel/module/tree_lookup.o
39644 4652 104 44400 ad70 (TOTALS)
- Aaron added module unload taint tracking (MODULE_UNLOAD_TAINT_TRACKING),
to enable tracking unloaded modules which did taint the kernel.
- Christophe Leroy added CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC
which lets architectures to request having modules data in vmalloc
area instead of module area. There are three reasons why an
architecture might want this:
a) On some architectures (like book3s/32) it is not possible to
protect against execution on a page basis. The exec stuff can be
mapped by different arch segment sizes (on book3s/32 that is 256M
segments). By default the module area is in an Exec segment while
vmalloc area is in a NoExec segment. Using vmalloc lets you muck
with module data as NoExec on those architectures whereas before
you could not.
b) By pushing more module data to vmalloc you also increase the
probability of module text to remain within a closer distance
from kernel core text and this reduces trampolines, this has been
reported on arm first and powerpc folks are following that lead.
c) Free'ing module_alloc() (Exec by default) area leaves this
exposed as Exec by default, some architectures have some security
enhancements to set this as NoExec on free, and splitting module
data with text let's future generic special allocators be added
to the kernel without having developers try to grok the tribal
knowledge per arch. Work like Rick Edgecombe's permission vmalloc
interface [0] becomes easier to address over time.
[0] https://lore.kernel.org/lkml/20201120202426.18009-1-rick.p.edgecombe@intel.com/#r
- Masahiro Yamada's symbol search enhancements
* tag 'modules-5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux: (33 commits)
module: merge check_exported_symbol() into find_exported_symbol_in_section()
module: do not binary-search in __ksymtab_gpl if fsa->gplok is false
module: do not pass opaque pointer for symbol search
module: show disallowed symbol name for inherit_taint()
module: fix [e_shstrndx].sh_size=0 OOB access
module: Introduce module unload taint tracking
module: Move module_assert_mutex_or_preempt() to internal.h
module: Make module_flags_taint() accept a module's taints bitmap and usable outside core code
module.h: simplify MODULE_IMPORT_NS
powerpc: Select ARCH_WANTS_MODULES_DATA_IN_VMALLOC on book3s/32 and 8xx
module: Remove module_addr_min and module_addr_max
module: Add CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC
module: Introduce data_layout
module: Prepare for handling several RB trees
module: Always have struct mod_tree_root
module: Rename debug_align() as strict_align()
module: Rework layout alignment to avoid BUG_ON()s
module: Move module_enable_x() and frob_text() in strict_rwx.c
module: Make module_enable_x() independent of CONFIG_ARCH_HAS_STRICT_MODULE_RWX
module: Move version support into a separate file
...
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