16e82be454
This patch reinstates a fix for PowerPC with regard to making VDSO calls while receiving a signal, and subsequently crashing. The crash happens because certain VDSO calls can modify the r30 register, which is where g is stored. This change was reverted for PowerPC because r30 is supposed to be a non-volatile register. This is true, but that only makes a guarantee across function calls, but not "within" a function call. This patch was seemingly fine before because the Linux kernel still had hand rolled assembly VDSO function calls, however with a recent change to C function calls it seems the compiler used can generate instructions which temporarily clobber r30. This means that when we receive a signal during one of these calls the value of r30 will not be the g as the runtime expects, causing a segfault. You can see from this assembly dump how the register is clobbered during the call: (the following is from a 5.13rc2 kernel) ``` Dump of assembler code for function __cvdso_clock_gettime_data: 0x00007ffff7ff0700 <+0>: cmplwi r4,15 0x00007ffff7ff0704 <+4>: bgt 0x7ffff7ff07f0 <__cvdso_clock_gettime_data+240> 0x00007ffff7ff0708 <+8>: li r9,1 0x00007ffff7ff070c <+12>: slw r9,r9,r4 0x00007ffff7ff0710 <+16>: andi. r10,r9,2179 0x00007ffff7ff0714 <+20>: beq 0x7ffff7ff0810 <__cvdso_clock_gettime_data+272> 0x00007ffff7ff0718 <+24>: rldicr r10,r4,4,59 0x00007ffff7ff071c <+28>: lis r9,32767 0x00007ffff7ff0720 <+32>: std r30,-16(r1) 0x00007ffff7ff0724 <+36>: std r31,-8(r1) 0x00007ffff7ff0728 <+40>: add r6,r3,r10 0x00007ffff7ff072c <+44>: ori r4,r9,65535 0x00007ffff7ff0730 <+48>: lwz r8,0(r3) 0x00007ffff7ff0734 <+52>: andi. r9,r8,1 0x00007ffff7ff0738 <+56>: bne 0x7ffff7ff07d0 <__cvdso_clock_gettime_data+208> 0x00007ffff7ff073c <+60>: lwsync 0x00007ffff7ff0740 <+64>: mftb r30 <---- RIGHT HERE => 0x00007ffff7ff0744 <+68>: ld r12,40(r6) ``` What I believe is happening is that the kernel changed the PowerPC VDSO calls to use standard C calls instead of using hand rolled assembly. The hand rolled assembly calls never touched r30, so this change was safe to roll back. That does not seem to be the case anymore as on the 5.13rc2 kernel the compiler *is* generating assembly which modifies r30, making this change again unsafe and causing a crash when the program receives a signal during these calls (which will happen often due to async preempt). This change happened here: https://lwn.net/ml/linux-kernel/235e5571959cfa89ced081d7e838ed5ff38447d2.1601365870.git.christophe.leroy@csgroup.eu/. I realize this was reverted due to unexplained hangs in PowerPC builders, but I think we should reinstate this change and investigate those issues separately: |
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