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Date: Tue, 25 Aug 2020 11:50:19 -0700 From: Kyle Huey <me@...ehuey.com> To: Andy Lutomirski <luto@...nel.org> Cc: "H. Peter Anvin" <hpa@...or.com>, "Robert O'Callahan" <robert@...llahan.org>, "Bae, Chang Seok" <chang.seok.bae@...el.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Andi Kleen <ak@...ux.intel.com>, "Shankar, Ravi V" <ravi.v.shankar@...el.com>, LKML <linux-kernel@...r.kernel.org>, "Hansen, Dave" <dave.hansen@...el.com> Subject: Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system On Tue, Aug 25, 2020 at 10:31 AM Kyle Huey <me@...ehuey.com> wrote: > > On Tue, Aug 25, 2020 at 9:46 AM Andy Lutomirski <luto@...nel.org> wrote: > > > > On Tue, Aug 25, 2020 at 9:32 AM Kyle Huey <me@...ehuey.com> wrote: > > > > > > On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <luto@...capital.net> wrote: > > > > I don’t like this at all. Your behavior really shouldn’t depend on > > > > whether the new instructions are available. Also, some day I would > > > > like to change Linux to have the new behavior even if FSGSBASE > > > > instructions are not available, and this will break rr again. (The > > > > current !FSGSBASE behavior is an ugly optimization of dubious value. > > > > I would not go so far as to describe it as correct.) > > > > > > Ok. > > > > > > > I would suggest you do one of the following things: > > > > > > > > 1. Use int $0x80 directly to load 32-bit regs into a child. This > > > > might dramatically simplify your code and should just do the right > > > > thing. > > > > > > I don't know what that means. > > > > This is untested, but what I mean is: > > > > static int ptrace32(int req, pid_t pid, int addr, int data) { > > int ret; > > /* new enough kernels won't clobber r8, etc. */ > > asm volatile ("int $0x80" : "=a" (ret) : "a" (26 /* ptrace */), "b" > > (req), "c" (pid), "d" (addr), "S" (data) : "flags", "r8", "r9", "r10", > > "r11"); > > return ret; > > } > > > > with a handful of caveats: > > > > - This won't compile with -fPIC, I think. Instead you'll need to > > write a little bit of asm to set up and restore ebx yourself. gcc is > > silly like this. > > > > - Note that addr is an int. You'll need to mmap(..., MAP_32BIT, ...) > > to get a buffer that can be pointed to with an int. > > > > The advantage is that this should work on all kernels that support > > 32-bit mode at all. > > > > > > > > > 2. Something like your patch but make it unconditional. > > > > > > > > 3. Ask for, and receive, real kernel support for setting FS and GS in > > > > the way that 32-bit code expects. > > > > > > I think the easiest way forward for us would be a PTRACE_GET/SETREGSET > > > like operation that operates on the regsets according to the > > > *tracee*'s bitness (rather than the tracer, as it works currently). > > > Does that sound workable? > > > > > > > Strictly speaking, on Linux, there is no unified concept of a task's > > bitness, so "set all these registers according to the target's > > bitness" is not well defined. We could easily give you a > > PTRACE_SETREGS_X86_32, etc, though. > > In the process of responding to this I spent some time doing code > inspection and discovered a subtlety in the ptrace API that I was > previously unaware of. PTRACE_GET/SETREGS use the regset views > corresponding to the tracer but PTRACE_GET/SETREGSET use the regset > views corresponding to the tracee. This means it is possible for us > today to set FS/GS "the old way" with a 64 bit tracer/32 bit tracee > combo, as long as we use PTRACE_SETREGSET with NT_PRSTATUS instead of > PTRACE_SETREGS. Alright I reverted the previous changes and switched us to use PTRACE_SETREGSET with NT_PRSTATUS[0] and our 32 bit tests pass again. I assume this behavior will remain unchanged indefinitely even when the fs/gsbase manipulation instructions are not available since the 32 bit user_regs_struct can't grow? - Kyle [0] https://github.com/mozilla/rr/commit/5c12d5f9ab77e526f852cbca82f454a42e3a6e30#diff-b509a7939392c11bb3c517b00da4526fL1447 (most of the rest of this commit is fixing our *emulation* of PTRACE_GETREGSET/PTRACE_SETREGSET which was not respecting the tracee's 32 vs 64 bit stauts).
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