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SubjectRe: [PATCH v8 5/8] powerpc/vdso: Prepare for switching VDSO to generic C implementation.
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On 07/15/2020 01:04 AM, Michael Ellerman wrote:
> 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.

Indeed, 32-bit doesn't have a redzone, so I believe it needs a stack
frame whenever it has anything to same. Here is what I have in libc.so:

000fbb60 <__clock_gettime>:
fbb60: 94 21 ff e0 stwu r1,-32(r1)
fbb64: 7c 08 02 a6 mflr r0
fbb68: 48 09 75 c9 bl 193130 <_IO_stdout_+0x24b0>
fbb6c: 93 c1 00 18 stw r30,24(r1)
fbb70: 7f c8 02 a6 mflr r30
fbb74: 90 01 00 24 stw r0,36(r1)
fbb78: 93 81 00 10 stw r28,16(r1)
fbb7c: 93 a1 00 14 stw r29,20(r1)
fbb80: 83 9e fc 98 lwz r28,-872(r30)
fbb84: 93 e1 00 1c stw r31,28(r1)
fbb88: 80 1c 00 00 lwz r0,0(r28)
fbb8c: 83 82 8f f4 lwz r28,-28684(r2)
fbb90: 7c 7f 1b 78 mr r31,r3
fbb94: 7c 00 e2 79 xor. r0,r0,r28
fbb98: 7c 9d 23 78 mr r29,r4
fbb9c: 41 82 00 40 beq fbbdc <__clock_gettime+0x7c>
fbba0: 7c 09 03 a6 mtctr r0
fbba4: 4e 80 04 21 bctrl
fbba8: 7c 00 00 26 mfcr r0
fbbac: 74 1c 10 00 andis. r28,r0,4096
fbbb0: 40 82 00 24 bne fbbd4 <__clock_gettime+0x74>
fbbb4: 80 01 00 24 lwz r0,36(r1)
fbbb8: 83 81 00 10 lwz r28,16(r1)
fbbbc: 7c 08 03 a6 mtlr r0
fbbc0: 83 a1 00 14 lwz r29,20(r1)
fbbc4: 83 c1 00 18 lwz r30,24(r1)
fbbc8: 83 e1 00 1c lwz r31,28(r1)
fbbcc: 38 21 00 20 addi r1,r1,32
fbbd0: 4e 80 00 20 blr
...
193130: 4e 80 00 21 blrl

But I guess if a prog has a way to avoid having anything to save, we may
face the same issue. So lets create two frames:

diff --git a/arch/powerpc/include/asm/vdso/gettimeofday.h
b/arch/powerpc/include/asm/vdso/gettimeofday.h
index a0712a6e80d9..0b6fa245d54e 100644
--- a/arch/powerpc/include/asm/vdso/gettimeofday.h
+++ b/arch/powerpc/include/asm/vdso/gettimeofday.h
@@ -10,6 +10,7 @@
.cfi_startproc
PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
mflr r0
+ PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
.cfi_register lr, r0
PPC_STL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
get_datapage r5, r0
@@ -19,7 +20,7 @@
cmpwi r3, 0
mtlr r0
.cfi_restore lr
- addi r1, r1, STACK_FRAME_OVERHEAD
+ addi r1, r1, 2 * STACK_FRAME_OVERHEAD
crclr so
beqlr+
crset so
@@ -32,6 +33,7 @@
.cfi_startproc
PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
mflr r0
+ PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
.cfi_register lr, r0
PPC_STL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
get_datapage r4, r0
@@ -41,7 +43,7 @@
crclr so
mtlr r0
.cfi_restore lr
- addi r1, r1, STACK_FRAME_OVERHEAD
+ addi r1, r1, 2 * STACK_FRAME_OVERHEAD
blr
.cfi_endproc
.endm


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

I see no code in the generated vdso64.so doing anything with r2, but if
you think that's needed, just let's do it:

commit 5a704d89bd5d7aac39194fb4c775f406905bf0a4
Author: Christophe Leroy <christophe.leroy@csgroup.eu>
Date: Tue Aug 4 06:31:48 2020 +0000
Save and restore GOT

diff --git a/arch/powerpc/include/asm/vdso/gettimeofday.h
b/arch/powerpc/include/asm/vdso/gettimeofday.h
index 0b6fa245d54e..fa774086173b 100644
--- a/arch/powerpc/include/asm/vdso/gettimeofday.h
+++ b/arch/powerpc/include/asm/vdso/gettimeofday.h
@@ -13,10 +13,16 @@
PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
.cfi_register lr, r0
PPC_STL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
+#ifdef CONFIG_PPC64
+ PPC_STL r2, STACK_FRAME_OVERHEAD + STK_GOT(r1)
+#endif
get_datapage r5, r0
addi r5, r5, VDSO_DATA_OFFSET
bl \funct
PPC_LL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
+#ifdef CONFIG_PPC64
+ PPC_LL r2, STACK_FRAME_OVERHEAD + STK_GOT(r1)
+#endif
cmpwi r3, 0
mtlr r0
.cfi_restore lr
@@ -36,10 +42,16 @@
PPC_STLU r1, -STACK_FRAME_OVERHEAD(r1)
.cfi_register lr, r0
PPC_STL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
+#ifdef CONFIG_PPC64
+ PPC_STL r2, STACK_FRAME_OVERHEAD + STK_GOT(r1)
+#endif
get_datapage r4, r0
addi r4, r4, VDSO_DATA_OFFSET
bl \funct
PPC_LL r0, STACK_FRAME_OVERHEAD + PPC_LR_STKOFF(r1)
+#ifdef CONFIG_PPC64
+ PPC_LL r2, STACK_FRAME_OVERHEAD + STK_GOT(r1)
+#endif
crclr so
mtlr r0
.cfi_restore lr

Christophe

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