Messages in this thread | | | Subject | Re: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80 | From | Denys Vlasenko <> | Date | Tue, 17 Apr 2018 17:00:43 +0200 |
| |
On 04/17/2018 04:36 PM, Andy Lutomirski wrote: > 32-bit user code that uses int $80 doesn't care about r8-r11. There is, > however, some 64-bit user code that intentionally uses int $0x80 to > invoke 32-bit system calls. From what I've seen, basically all such > code assumes that r8-r15 are all preserved, but the kernel clobbers > r8-r11. Since I doubt that there's any code that depends on int $0x80 > zeroing r8-r11, change the kernel to preserve them. > > I suspect that very little user code is broken by the old clobber, > since r8-r11 are only rarely allocated by gcc, and they're clobbered > by function calls, so they only way we'd see a problem is if the > same function that invokes int $0x80 also spills something important > to one of these registers. > > The current behavior seems to date back to the historical commit > "[PATCH] x86-64 merge for 2.6.4". Before that, all regs were > preserved. I can't find any explanation of why this change was made.
This means that the new behavior is there for some 8 years already. Whoever was impacted by it, probably already switched to the new ABI.
Current ABI is "weaker", it allows kernel to save fewer registers.
Which is generally a good thing, since saving/restoring things cost cycles, and sometimes painful on entry paths where you may desperately need a scratch register or two. (Recall this one? - ... movq %rsp, PER_CPU_VAR(rsp_scratch) movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp /* Construct struct pt_regs on stack */ pushq $__USER_DS /* pt_regs->ss */ pushq PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */ ... wouldn't it be _great_ if one of GPRs would be available here to hold userspace %rsp? )
If userspace needs some registers saved, it's trivial for it to have:
push reg1 push reg2 int 0x80 pop reg2 pop reg1
OTOH if userspace _does not_ need some registers saved, but they are defined as saved by the entrypoint ABI, then save/restore work is done every time, even when not needed.
Thus, I propose to retain the current behavior.
| |