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SubjectRe: [PATCH v8 5/8] powerpc/vdso: Prepare for switching VDSO to generic C implementation.
Date
Christophe Leroy <christophe.leroy@c-s.fr> 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.

This is an example from a statically built program that calls
clock_gettime():

0000000010030cb0 <__clock_gettime>:
10030cb0: 0e 10 40 3c lis r2,4110
10030cb4: 00 7a 42 38 addi r2,r2,31232
10030cb8: a6 02 08 7c mflr r0
10030cbc: ff ff 22 3d addis r9,r2,-1
10030cc0: 58 6d 29 39 addi r9,r9,27992
10030cc4: f0 ff c1 fb std r30,-16(r1) <-- redzone store
10030cc8: 78 23 9e 7c mr r30,r4
10030ccc: f8 ff e1 fb std r31,-8(r1) <-- redzone store
10030cd0: 78 1b 7f 7c mr r31,r3
10030cd4: 10 00 01 f8 std r0,16(r1) <-- save LR to caller's frame
10030cd8: 00 00 09 e8 ld r0,0(r9)
10030cdc: 00 00 20 2c cmpdi r0,0
10030ce0: 50 00 82 41 beq 10030d30 <__clock_gettime+0x80>
10030ce4: a6 03 09 7c mtctr r0
10030ce8: 21 04 80 4e bctrl <-- vdso call
10030cec: 26 00 00 7c mfcr r0
10030cf0: 00 10 09 74 andis. r9,r0,4096
10030cf4: 78 1b 69 7c mr r9,r3
10030cf8: 28 00 82 40 bne 10030d20 <__clock_gettime+0x70>
10030cfc: b4 07 23 7d extsw r3,r9
10030d00: 10 00 01 e8 ld r0,16(r1) <-- load saved LR, since clobbered by the VDSO
10030d04: f0 ff c1 eb ld r30,-16(r1)
10030d08: f8 ff e1 eb ld r31,-8(r1)
10030d0c: a6 03 08 7c mtlr r0 <-- restore LR
10030d10: 20 00 80 4e blr <-- jumps to 10030cec


I'm kind of confused how it worked for you on 32-bit.

There's also no code to load/restore the TOC pointer on BE, which I
think we'll need to handle.

cheers

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