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Jens Remus
cea5589e95
Since commit778666df60("s390: compile relocatable kernel without -fPIE") and commit00cda11d3b("s390: Compile kernel with -fPIC and link with -no-pie") the kernel on s390x may have a Global Offset Table (GOT) whose entries are adjusted for KASLR in kaslr_adjust_got(). The GOT may contain entries for undefined weak symbols that resolved to zero. That is the resulting GOT entry value is zero. Adjusting those entries unconditionally in kaslr_adjust_got() is wrong. Otherwise the following sample code would erroneously assume foo to be defined, due to the adjustment changing the zero-value to a non-zero one: extern int foo __attribute__((weak)); if (*foo) /* foo is defined [or undefined and erroneously adjusted] */ The vmlinux build at commit00cda11d3b("s390: Compile kernel with -fPIC and link with -no-pie") with defconfig actually had two GOT entries for the undefined weak symbols __start_BTF and __stop_BTF: $ objdump -tw vmlinux | grep -F "*UND*" 0000000000000000 w *UND* 0000000000000000 __stop_BTF 0000000000000000 w *UND* 0000000000000000 __start_BTF $ readelf -rw vmlinux | grep -E "R_390_GOTENT +0{16}" 000000345760 2776a0000001a R_390_GOTENT 0000000000000000 __stop_BTF + 2 000000345766 2d5480000001a R_390_GOTENT 0000000000000000 __start_BTF + 2 The s390-specific vmlinux linker script sets the section start to __START_KERNEL, which is currently defined as 0x100000 on s390x. Access to lowcore is performed via a pointer of 0 and not a symbol in a section starting at 0. The first 64K are reserved for the loader on s390x. Thus it is safe to assume that __START_KERNEL will never be 0. As a result there cannot be any defined symbols resolving to zero in the kernel. Note that the first three GOT entries are reserved for the dynamic loader on s390x. [1] In the kernel they are zero. Therefore no extra handling is required to skip these. Skip adjusting GOT entries with a value of zero in kaslr_adjust_got(). While at it update the comment when a GOT exists on s390x. Since commit00cda11d3b("s390: Compile kernel with -fPIC and link with -no-pie") it no longer only exists when compiling with Clang, but also with GCC. [1]: s390x ELF ABI, section "Global Offset Table", https://github.com/IBM/s390x-abi/releases Fixes:778666df60("s390: compile relocatable kernel without -fPIE") Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com> Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com> Acked-by: Alexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: Jens Remus <jremus@linux.ibm.com> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
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 reStructuredText 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.