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Paul Burton 9b8a065de9 MIPS: Avoid VDSO ABI breakage due to global register variable ... commit bbcc5672b0063b0e9d65dc8787a4f09c3b5bb5cc upstream. Declaring __current_thread_info as a global register variable has the effect of preventing GCC from saving & restoring its value in cases where the ABI would typically do so. To quote GCC documentation: > If the register is a call-saved register, call ABI is affected: the > register will not be restored in function epilogue sequences after the > variable has been assigned. Therefore, functions cannot safely return > to callers that assume standard ABI. When our position independent VDSO is built for the n32 or n64 ABIs all functions it exposes should be preserving the value of $gp/$28 for their caller, but in the presence of the __current_thread_info global register variable GCC stops doing so & simply clobbers $gp/$28 when calculating the address of the GOT. In cases where the VDSO returns success this problem will typically be masked by the caller in libc returning & restoring $gp/$28 itself, but that is by no means guaranteed. In cases where the VDSO returns an error libc will typically contain a fallback path which will now fail (typically with a bad memory access) if it attempts anything which relies upon the value of $gp/$28 - eg. accessing anything via the GOT. One fix for this would be to move the declaration of __current_thread_info inside the current_thread_info() function, demoting it from global register variable to local register variable & avoiding inadvertently creating a non-standard calling ABI for the VDSO. Unfortunately this causes issues for clang, which doesn't support local register variables as pointed out by commit fe92da0f35 ("MIPS: Changed current_thread_info() to an equivalent supported by both clang and GCC") which introduced the global register variable before we had a VDSO to worry about. Instead, fix this by continuing to use the global register variable for the kernel proper but declare __current_thread_info as a simple extern variable when building the VDSO. It should never be referenced, and will cause a link error if it is. This resolves the calling convention issue for the VDSO without having any impact upon the build of the kernel itself for either clang or gcc. Signed-off-by: Paul Burton <paulburton@kernel.org> Fixes: ebb5e78cc6 ("MIPS: Initial implementation of a VDSO") Reported-by: Jason A. Donenfeld <Jason@zx2c4.com> Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com> Tested-by: Jason A. Donenfeld <Jason@zx2c4.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christian Brauner <christian.brauner@canonical.com> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> Cc: <stable@vger.kernel.org> # v4.4+ Cc: linux-mips@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2020-01-09 10:18:59 +01:00
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asm	MIPS: Avoid VDSO ABI breakage due to global register variable	2020-01-09 10:18:59 +01:00
uapi/asm	MIPS: elf_hwcap: Export userspace ASEs	2019-10-17 13:45:32 -07:00