[PATCH] powerpc/vdso: Don't use r30 to avoid breaking Go lang
Michael Ellerman
mpe at ellerman.id.au
Mon Aug 2 21:27:33 AEST 2021
Michael Ellerman <mpe at ellerman.id.au> writes:
> The Go runtime uses r30 for some special value called 'g'. It assumes
> that value will remain unchanged even when calling VDSO functions.
> Although r30 is non-volatile across function calls, the callee is free
> to use it, as long as the callee saves the value and restores it before
> returning.
>
> It used to be true by accident that the VDSO didn't use r30, because the
> VDSO was hand-written asm. When we switched to building the VDSO from C
> the compiler started using r30, at least in some builds, leading to
> crashes in Go. eg:
>
> ~/go/src$ ./all.bash
> Building Go cmd/dist using /usr/lib/go-1.16. (go1.16.2 linux/ppc64le)
> Building Go toolchain1 using /usr/lib/go-1.16.
> go build os/exec: /usr/lib/go-1.16/pkg/tool/linux_ppc64le/compile: signal: segmentation fault
> go build reflect: /usr/lib/go-1.16/pkg/tool/linux_ppc64le/compile: signal: segmentation fault
> go tool dist: FAILED: /usr/lib/go-1.16/bin/go install -gcflags=-l -tags=math_big_pure_go compiler_bootstrap bootstrap/cmd/...: exit status 1
>
> There are patches in flight to fix Go[1], but until they are released
> and widely deployed we can workaround it in the VDSO by avoiding use of
> r30.
>
> Note this only works with GCC, clang does not support -ffixed-rN.
>
> 1: https://go-review.googlesource.com/c/go/+/328110
>
> Fixes: ab037dd87a2f ("powerpc/vdso: Switch VDSO to generic C implementation.")
> Cc: stable at vger.kernel.org # v5.11+
In practice, with GCC 10.3.0, that commit doesn't result in r30 being
used by the compiler.
It's commit 74205b3fc2ef ("powerpc/vdso: Add support for time
namespaces"), which went into v5.13-rc1, which causes r30 to be used in
__c_kernel_clock_gettime():
00000000000006e0 <__c_kernel_clock_gettime>:
6e0: 0f 00 03 28 cmplwi r3,15
6e4: ec 00 81 41 bgt 7d0 <__c_kernel_clock_gettime+0xf0>
6e8: 01 00 20 39 li r9,1
6ec: 30 18 29 7d slw r9,r9,r3
6f0: 83 08 2a 71 andi. r10,r9,2179
6f4: fc 00 82 41 beq 7f0 <__c_kernel_clock_gettime+0x110>
6f8: e4 26 63 78 rldicr r3,r3,4,59
6fc: ff 7f 20 3d lis r9,32767
700: f0 ff c1 fb std r30,-16(r1)
704: f8 ff e1 fb std r31,-8(r1)
708: 14 1a c5 7c add r6,r5,r3
70c: ff ff 2b 61 ori r11,r9,65535
710: 00 00 05 81 lwz r8,0(r5)
714: 01 00 09 71 andi. r9,r8,1
718: 98 00 82 40 bne 7b0 <__c_kernel_clock_gettime+0xd0>
71c: ac 04 20 7c lwsync
720: a6 42 cc 7f mftb r30
cheers
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