The modversion symbol CRCs are emitted as ELF symbols, which allows us
to easily populate the kcrctab sections by relying on the linker to
associate each kcrctab slot with the correct value.
This has a couple of downsides:
- Given that the CRCs are treated as memory addresses, we waste 4 bytes
for each CRC on 64 bit architectures,
- On architectures that support runtime relocation, a R_<arch>_RELATIVE
relocation entry is emitted for each CRC value, which identifies it
as a quantity that requires fixing up based on the actual runtime
load offset of the kernel. This results in corrupted CRCs unless we
explicitly undo the fixup (and this is currently being handled in the
core module code)
- Such runtime relocation entries take up 24 bytes of __init space
each, resulting in a x8 overhead in [uncompressed] kernel size for
CRCs.
Switching to explicit 32 bit values on 64 bit architectures fixes most
of these issues, given that 32 bit values are not treated as quantities
that require fixing up based on the actual runtime load offset. Note
that on some ELF64 architectures [such as PPC64], these 32-bit values
are still emitted as [absolute] runtime relocatable quantities, even if
the value resolves to a build time constant. Since relative relocations
are always resolved at build time, this patch enables MODULE_REL_CRCS on
powerpc when CONFIG_RELOCATABLE=y, which turns the absolute CRC
references into relative references into .rodata where the actual CRC
value is stored.
So redefine all CRC fields and variables as u32, and redefine the
__CRC_SYMBOL() macro for 64 bit builds to emit the CRC reference using
inline assembler (which is necessary since 64-bit C code cannot use
32-bit types to hold memory addresses, even if they are ultimately
resolved using values that do not exceed 0xffffffff). To avoid
potential problems with legacy 32-bit architectures using legacy
toolchains, the equivalent C definition of the kcrctab entry is retained
for 32-bit architectures.
Note that this mostly reverts commit d4703aefdb ("module: handle ppc64
relocating kcrctabs when CONFIG_RELOCATABLE=y")
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This reverts commit 8ab2ae655b.
I loved that commit because of how it explained what the problem with
newer versions of binutils were, but the actual patch itself turns out
to not work very well.
It has two problems:
- a zero CRC value isn't actually right. It happens to work for the
case where both sides of the equation fail at giving the symbol a
crc, but there are cases where the users of the exported symbol get
the right crc (due to seeing the C declarations), but the actual
exporting itself does not (due to the whole weak asm symbol issue).
So then the module load fails after all - we did have a crc for the
symbol, but we couldn't match it with the loaded module.
- it seems that the alpha assembler has special semantics for the
'.set' directive, and on alpha it doesn't actually set the value of
the specified symbol at all, it is instead used to set various
assembly modes (eg ".set noat" and ".set noreorder").
So using ".set" to set the symbol value would just cause build
failures on alpha.
I'm sure we'll find some other workaround for these issues (hopefully
that involves getting rid of modversions entirely some day, but people
are also talking about just using smarter tools). But for now we'll
just fall back on commit faaae2a581 ("Re-enable CONFIG_MODVERSIONS in
a slightly weaker form") that just let's a missing crc through.
Reported-by: Jan Stancek <jstancek@redhat.com>
Reported-by: Philip Müller <philm@manjaro.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
With binutils-2.26 and before, a weak missing symbol was kept during the
final link, and a missing CRC for an export would lead to that CRC being
treated as zero implicitly. With binutils-2.27, the crc symbol gets
dropped, and any module trying to use it will fail to load.
This sets the weak CRC symbol to zero explicitly, making it defined in
vmlinux, which in turn lets us load the modules referring to that CRC.
The comment above the __CRC_SYMBOL macro suggests that this was always
the intention, although it also seems that all symbols defined in C have
a correct CRC these days, and only the exports that are now done in
assembly need this.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Adam Borowski <kilobyte@angband.pl>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
The use of config_enabled() is ambiguous. For config options,
IS_ENABLED(), IS_REACHABLE(), etc. will make intention clearer.
Sometimes config_enabled() has been used for non-config options because
it is useful to check whether the given symbol is defined or not.
I have been tackling on deprecating config_enabled(), and now is the
time to finish this work.
Some new users have appeared for v4.9-rc1, but it is trivial to replace
them:
- arch/x86/mm/kaslr.c
replace config_enabled() with IS_ENABLED() because
CONFIG_X86_ESPFIX64 and CONFIG_EFI are boolean.
- include/asm-generic/export.h
replace config_enabled() with __is_defined().
Then, config_enabled() can be removed now.
Going forward, please use IS_ENABLED(), IS_REACHABLE(), etc. for config
options, and __is_defined() for non-config symbols.
Link: http://lkml.kernel.org/r/1476616078-32252-1-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michal Marek <mmarek@suse.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Add asm-usable variants of EXPORT_SYMBOL/EXPORT_SYMBOL_GPL. This
commit just adds the default implementation; most of the architectures
can simply add export.h to asm/Kbuild and start using <asm/export.h>
from assembler. The rest needs to have their <asm/export.h> define
everal macros and then explicitly include <asm-generic/export.h>
One area where the things might diverge from default is the alignment;
normally it's 8 bytes on 64bit targets and 4 on 32bit ones, both for
unsigned long and for struct kernel_symbol. Unfortunately, amd64 and
m68k are unusual - m68k aligns to 2 bytes (for both) and amd64 aligns
struct kernel_symbol to 16 bytes. For those we'll need asm/export.h to
override the constants used by generic version - KSYM_ALIGN and KCRC_ALIGN
for kernel_symbol and unsigned long resp. And no, __alignof__ would not
do the trick - on amd64 __alignof__ of struct kernel_symbol is 8, not 16.
More serious source of unpleasantness is treatment of function
descriptors on architectures that have those. Things like ppc64,
parisc, ia64, etc. need more than the address of the first insn to
call an arbitrary function. As the result, their representation of
pointers to functions is not the typical "address of the entry point" -
it's an address of a small static structure containing all the required
information (including the entry point, of course). Sadly, the asm-side
conventions differ in what the function name refers to - entry point or
the function descriptor. On ppc64 we do the latter;
bar: .quad foo
is what void (*bar)(void) = foo; turns into and the rare places where
we need to explicitly work with the label of entry point are dealt with
as DOTSYM(foo). For our purposes it's ideal - generic macros are usable.
However, parisc would have foo and P%foo used for label of entry point
and address of the function descriptor and
bar: .long P%foo
woudl be used instead. ia64 goes similar to parisc in that respect,
except that there it's @fptr(foo) rather than P%foo. Such architectures
need to define KSYM_FUNC that would turn a function name into whatever
is needed to refer to function descriptor.
What's more, on such architectures we need to know whether we are exporting
a function or an object - in assembler we have to tell that explicitly, to
decide whether we want EXPORT_SYMBOL(foo) produce e.g.
__ksymtab_foo: .quad foo
or
__ksymtab_foo: .quad @fptr(foo)
For that reason we introduce EXPORT_DATA_SYMBOL{,_GPL}(), to be used for
exports of data objects. On normal architectures it's the same thing
as EXPORT_SYMBOL{,_GPL}(), but on parisc-like ones they differ and the
right one needs to be used. Most of the exports are functions, so we
keep EXPORT_SYMBOL for those...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
