Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes: > Michael Ellerman <mpe@xxxxxxxxxxxxxx> a écrit : > >> Christophe Leroy <christophe.leroy@xxxxxx> writes: >>> Prepare for switching VDSO to generic C implementation in following >>> patch. Here, we: >>> - Modify __get_datapage() to take an offset >>> - Prepare the helpers to call the C VDSO functions >>> - Prepare the required callbacks for the C VDSO functions >>> - Prepare the clocksource.h files to define VDSO_ARCH_CLOCKMODES >>> - Add the C trampolines to the generic C VDSO functions >>> >>> powerpc is a bit special for VDSO as well as system calls in the >>> way that it requires setting CR SO bit which cannot be done in C. >>> Therefore, entry/exit needs to be performed in ASM. >>> >>> Implementing __arch_get_vdso_data() would clobber the link register, >>> requiring the caller to save it. As the ASM calling function already >>> has to set a stack frame and saves the link register before calling >>> the C vdso function, retriving the vdso data pointer there is lighter. >> ... >> >>> diff --git a/arch/powerpc/include/asm/vdso/gettimeofday.h >>> b/arch/powerpc/include/asm/vdso/gettimeofday.h >>> new file mode 100644 >>> index 000000000000..4452897f9bd8 >>> --- /dev/null >>> +++ b/arch/powerpc/include/asm/vdso/gettimeofday.h >>> @@ -0,0 +1,175 @@ >>> +/* SPDX-License-Identifier: GPL-2.0 */ >>> +#ifndef __ASM_VDSO_GETTIMEOFDAY_H >>> +#define __ASM_VDSO_GETTIMEOFDAY_H >>> + >>> +#include <asm/ptrace.h> >>> + >>> +#ifdef __ASSEMBLY__ >>> + >>> +.macro cvdso_call funct >>> + .cfi_startproc >>> + PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1) >>> + mflr r0 >>> + .cfi_register lr, r0 >>> + PPC_STL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1) >> >> This doesn't work for me on ppc64(le) with glibc. >> >> glibc doesn't create a stack frame before making the VDSO call, so the >> store of r0 (LR) goes into the caller's frame, corrupting the saved LR, >> leading to an infinite loop. > > Where should it be saved if it can't be saved in the standard location ? As Michael pointed out, userspace doesn't treat the VDSO as a normal function call. In order to keep compatibility with existent software, LR would need to be saved on another stack frame. -- Tulio Magno
