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When CONFIG_THUMB2_KERNEL is enabled, modpost fails to detect some
types of section mismatches.
[test code]
#include <linux/init.h>
int __initdata foo;
int get_foo(void) { return foo; }
It is apparently a bad reference, but modpost does not report anything.
The test code above produces the following relocations.
Relocation section '.rel.text' at offset 0x1e8 contains 2 entries:
Offset Info Type Sym.Value Sym. Name
00000000 0000052f R_ARM_THM_MOVW_AB 00000000 .LANCHOR0
00000004 00000530 R_ARM_THM_MOVT_AB 00000000 .LANCHOR0
Currently, R_ARM_THM_MOVW_ABS_NC and R_ARM_THM_MOVT_ABS are just skipped.
Add code to handle them. I checked arch/arm/kernel/module.c to learn
how the offset is encoded in the instruction.
One more thing to note for Thumb instructions - the st_value is an odd
value, so you need to mask the bit 0 to get the offset. Otherwise, you
will get an off-by-one error in the nearest symbol look-up.
It is documented in "ELF for the ARM Architecture" [1]:
In addition to the normal rules for symbol values the following rules
shall also apply to symbols of type STT_FUNC:
* If the symbol addresses an Arm instruction, its value is the
address of the instruction (in a relocatable object, the offset
of the instruction from the start of the section containing it).
* If the symbol addresses a Thumb instruction, its value is the
address of the instruction with bit zero set (in a relocatable
object, the section offset with bit zero set).
* For the purposes of relocation the value used shall be the address
of the instruction (st_value & ~1).
[1]: https://github.com/ARM-software/abi-aa/blob/main/aaelf32/aaelf32.rst
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.