From: rostedt at goodmis.org (Steven Rostedt) Subject: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions Date: Tue, 7 May 2019 10:57:24 -0400 [thread overview] Message-ID: <20190507105724.02abe6f6@gandalf.local.home> (raw) In-Reply-To: <b4a24fbe906a438798870c5112cde8b2@AcuMS.aculab.com> On Tue, 7 May 2019 14:50:26 +0000 David Laight <David.Laight at ACULAB.COM> wrote: > From: Steven Rostedt > > Sent: 07 May 2019 14:14 > > On Tue, 7 May 2019 12:57:15 +0000 > > David Laight <David.Laight at ACULAB.COM> wrote: > The 'user' (ie the kernel code that needs to emulate the call) doesn't > write the data to the stack, just to some per-cpu location. > (Actually it could be on the stack at the other end of pt-regs.) > So you get to the 'register restore and iret' code with the stack unaltered. > It is then a SMOP to replace the %flags saved by the int3 with the %ip > saved by the int3, the %ip with the address of the function to call, > restore the flags (push and popf) and issue a ret.f to remove the %ip and %cs. How would you handle NMIs doing the same thing? Yes, the NMI handlers have breakpoints that will need to emulated calls as well. > > (Actually you need to add 4 to the callers %ip address to allow for the > difference between the size of int3 (hopefully 0xcc, not 0xcd 0x3).) > > > > > For 32bit 'the gap' happens naturally when building a 5 entry frame. Yes > > > > it is possible to build a 5 entry frame on top of the old 3 entry one, > > > > but why bother... > > > > > > Presumably there is 'horrid' code to generate the gap in 64bit mode? > > > (less horrid than 32bit, but still horrid?) > > > Or does it copy the entire pt_regs into a local stack frame and use > > > that for the iret? > > > > On x86_64, the gap is only done for int3 and nothing else, thus it is > > much less horrid. That's because x86_64 has a sane pt_regs storage for > > all exceptions. > > Well, in particular, it always loads %sp as part of the iret. > So you can create a gap and the cpu will remove it for you. > > In 64bit mode you could overwrite the %ss with the return address > to the caller restore %eax and %flags, push the function address > and use ret.n to jump to the function subtracting the right amount > from %esp. > > Actually that means you can do the following in both modes: > if not emulated_call_address then pop %ax; iret else > # assume kernel<->kernel return > push emulated_call_address; > push flags_saved_by_int3 > load %ax, return_address_from_iret > add %ax,#4 > store %ax, first_stack_location_written_by_int3 > load %ax, value_saved_by_int3_entry > popf > ret.n > > The ret.n discards everything from the %ax to the required return address. > So 'n' is the size of the int3 frame, so 12 for i386 and 40 for amd64. > > If the register restore (done just before this code) finished with > 'add %sp, sizeof *pt_regs' then the emulated_call_address can be > loaded in %ax from the other end of pt_regs. > > This all reminds me of fixing up the in-kernel faults that happen > when loading the user segment registers during 'return to user' > fault in kernel space. This all sounds much more complex and fragile than the proposed solution. Why would we do this over what is being proposed? -- Steve
WARNING: multiple messages have this Message-ID (diff)
From: rostedt@goodmis.org (Steven Rostedt) Subject: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions Date: Tue, 7 May 2019 10:57:24 -0400 [thread overview] Message-ID: <20190507105724.02abe6f6@gandalf.local.home> (raw) Message-ID: <20190507145724.yv7eIzE0bzc3MIDS7F0VcXRBe9pE9obahge307s1lWw@z> (raw) In-Reply-To: <b4a24fbe906a438798870c5112cde8b2@AcuMS.aculab.com> On Tue, 7 May 2019 14:50:26 +0000 David Laight <David.Laight@ACULAB.COM> wrote: > From: Steven Rostedt > > Sent: 07 May 2019 14:14 > > On Tue, 7 May 2019 12:57:15 +0000 > > David Laight <David.Laight@ACULAB.COM> wrote: > The 'user' (ie the kernel code that needs to emulate the call) doesn't > write the data to the stack, just to some per-cpu location. > (Actually it could be on the stack at the other end of pt-regs.) > So you get to the 'register restore and iret' code with the stack unaltered. > It is then a SMOP to replace the %flags saved by the int3 with the %ip > saved by the int3, the %ip with the address of the function to call, > restore the flags (push and popf) and issue a ret.f to remove the %ip and %cs. How would you handle NMIs doing the same thing? Yes, the NMI handlers have breakpoints that will need to emulated calls as well. > > (Actually you need to add 4 to the callers %ip address to allow for the > difference between the size of int3 (hopefully 0xcc, not 0xcd 0x3).) > > > > > For 32bit 'the gap' happens naturally when building a 5 entry frame. Yes > > > > it is possible to build a 5 entry frame on top of the old 3 entry one, > > > > but why bother... > > > > > > Presumably there is 'horrid' code to generate the gap in 64bit mode? > > > (less horrid than 32bit, but still horrid?) > > > Or does it copy the entire pt_regs into a local stack frame and use > > > that for the iret? > > > > On x86_64, the gap is only done for int3 and nothing else, thus it is > > much less horrid. That's because x86_64 has a sane pt_regs storage for > > all exceptions. > > Well, in particular, it always loads %sp as part of the iret. > So you can create a gap and the cpu will remove it for you. > > In 64bit mode you could overwrite the %ss with the return address > to the caller restore %eax and %flags, push the function address > and use ret.n to jump to the function subtracting the right amount > from %esp. > > Actually that means you can do the following in both modes: > if not emulated_call_address then pop %ax; iret else > # assume kernel<->kernel return > push emulated_call_address; > push flags_saved_by_int3 > load %ax, return_address_from_iret > add %ax,#4 > store %ax, first_stack_location_written_by_int3 > load %ax, value_saved_by_int3_entry > popf > ret.n > > The ret.n discards everything from the %ax to the required return address. > So 'n' is the size of the int3 frame, so 12 for i386 and 40 for amd64. > > If the register restore (done just before this code) finished with > 'add %sp, sizeof *pt_regs' then the emulated_call_address can be > loaded in %ax from the other end of pt_regs. > > This all reminds me of fixing up the in-kernel faults that happen > when loading the user segment registers during 'return to user' > fault in kernel space. This all sounds much more complex and fragile than the proposed solution. Why would we do this over what is being proposed? -- Steve
next prev parent reply other threads:[~2019-05-07 14:57 UTC|newest] Thread overview: 204+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <20190501202830.347656894@goodmis.org> 2019-05-01 20:28 ` [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions rostedt 2019-05-01 20:28 ` Steven Rostedt 2019-05-02 3:24 ` rostedt 2019-05-02 3:24 ` Steven Rostedt 2019-05-02 16:21 ` peterz 2019-05-02 16:21 ` Peter Zijlstra 2019-05-02 16:29 ` peterz 2019-05-02 16:29 ` Peter Zijlstra 2019-05-02 18:02 ` torvalds 2019-05-02 18:02 ` Linus Torvalds 2019-05-02 18:18 ` peterz 2019-05-02 18:18 ` Peter Zijlstra 2019-05-02 18:30 ` peterz 2019-05-02 18:30 ` Peter Zijlstra 2019-05-02 18:43 ` torvalds 2019-05-02 18:43 ` Linus Torvalds 2019-05-02 19:28 ` jikos 2019-05-02 19:28 ` Jiri Kosina 2019-05-02 20:25 ` luto 2019-05-02 20:25 ` Andy Lutomirski 2019-05-02 20:21 ` peterz 2019-05-02 20:21 ` Peter Zijlstra 2019-05-02 20:49 ` torvalds 2019-05-02 20:49 ` Linus Torvalds 2019-05-02 21:32 ` peterz 2019-05-02 21:32 ` Peter Zijlstra 2019-05-03 19:24 ` rostedt 2019-05-03 19:24 ` Steven Rostedt 2019-05-03 21:46 ` torvalds 2019-05-03 21:46 ` Linus Torvalds 2019-05-03 22:49 ` rostedt 2019-05-03 22:49 ` Steven Rostedt 2019-05-03 23:07 ` torvalds 2019-05-03 23:07 ` Linus Torvalds 2019-05-04 4:17 ` rostedt 2019-05-04 4:17 ` Steven Rostedt [not found] ` <CAHk-=wiuSFbv_rELND-BLWcP0GSZ0yF=xOAEcf61GE3bU9d=yg@mail.gmail.com> 2019-05-04 18:59 ` torvalds 2019-05-04 18:59 ` Linus Torvalds 2019-05-04 20:12 ` luto 2019-05-04 20:12 ` Andy Lutomirski 2019-05-04 20:28 ` torvalds 2019-05-04 20:28 ` Linus Torvalds 2019-05-04 20:36 ` torvalds 2019-05-04 20:36 ` Linus Torvalds 2019-05-03 22:55 ` luto 2019-05-03 22:55 ` Andy Lutomirski 2019-05-03 23:16 ` torvalds 2019-05-03 23:16 ` Linus Torvalds 2019-05-03 23:32 ` luto 2019-05-03 23:32 ` Andy Lutomirski 2019-05-02 22:52 ` rostedt 2019-05-02 22:52 ` Steven Rostedt 2019-05-02 23:31 ` rostedt 2019-05-02 23:31 ` Steven Rostedt 2019-05-02 23:50 ` rostedt 2019-05-02 23:50 ` Steven Rostedt 2019-05-03 1:51 ` [RFC][PATCH 1/2 v2] " rostedt 2019-05-03 1:51 ` Steven Rostedt 2019-05-03 9:29 ` [RFC][PATCH 1/2] " peterz 2019-05-03 9:29 ` Peter Zijlstra 2019-05-03 13:22 ` rostedt 2019-05-03 13:22 ` Steven Rostedt 2019-05-03 16:20 ` luto 2019-05-03 16:20 ` Andy Lutomirski 2019-05-03 16:31 ` rostedt 2019-05-03 16:31 ` Steven Rostedt 2019-05-03 16:35 ` peterz 2019-05-03 16:35 ` Peter Zijlstra 2019-05-03 16:44 ` luto 2019-05-03 16:44 ` Andy Lutomirski 2019-05-03 16:49 ` rostedt 2019-05-03 16:49 ` Steven Rostedt 2019-05-03 16:32 ` peterz 2019-05-03 16:32 ` Peter Zijlstra 2019-05-03 18:57 ` torvalds 2019-05-03 18:57 ` Linus Torvalds 2019-05-06 8:19 ` peterz 2019-05-06 8:19 ` Peter Zijlstra 2019-05-06 13:56 ` rostedt 2019-05-06 13:56 ` Steven Rostedt 2019-05-06 16:17 ` torvalds 2019-05-06 16:17 ` Linus Torvalds 2019-05-06 16:19 ` torvalds 2019-05-06 16:19 ` Linus Torvalds 2019-05-06 17:06 ` rostedt 2019-05-06 17:06 ` Steven Rostedt 2019-05-06 18:06 ` torvalds 2019-05-06 18:06 ` Linus Torvalds 2019-05-06 18:57 ` rostedt 2019-05-06 18:57 ` Steven Rostedt 2019-05-06 19:46 ` torvalds 2019-05-06 19:46 ` Linus Torvalds 2019-05-06 20:29 ` rostedt 2019-05-06 20:29 ` Steven Rostedt 2019-05-06 20:42 ` torvalds 2019-05-06 20:42 ` Linus Torvalds 2019-05-06 20:44 ` torvalds 2019-05-06 20:44 ` Linus Torvalds 2019-05-06 21:45 ` rostedt 2019-05-06 21:45 ` Steven Rostedt 2019-05-06 22:06 ` torvalds 2019-05-06 22:06 ` Linus Torvalds 2019-05-06 22:31 ` torvalds 2019-05-06 22:31 ` Linus Torvalds 2019-05-07 0:10 ` rostedt 2019-05-07 0:10 ` Steven Rostedt 2019-05-07 1:06 ` torvalds 2019-05-07 1:06 ` Linus Torvalds 2019-05-07 1:04 ` rostedt 2019-05-07 1:04 ` Steven Rostedt 2019-05-07 1:34 ` rostedt 2019-05-07 1:34 ` Steven Rostedt 2019-05-07 1:34 ` torvalds 2019-05-07 1:34 ` Linus Torvalds 2019-05-07 1:53 ` rostedt 2019-05-07 1:53 ` Steven Rostedt 2019-05-07 2:22 ` torvalds 2019-05-07 2:22 ` Linus Torvalds 2019-05-07 2:58 ` rostedt 2019-05-07 2:58 ` Steven Rostedt 2019-05-07 3:05 ` torvalds 2019-05-07 3:05 ` Linus Torvalds 2019-05-07 3:21 ` rostedt 2019-05-07 3:21 ` Steven Rostedt 2019-05-07 3:28 ` torvalds 2019-05-07 3:28 ` Linus Torvalds 2019-05-07 14:54 ` torvalds 2019-05-07 14:54 ` Linus Torvalds 2019-05-07 15:12 ` rostedt 2019-05-07 15:12 ` Steven Rostedt 2019-05-07 15:25 ` rostedt 2019-05-07 15:25 ` Steven Rostedt 2019-05-07 16:25 ` rostedt 2019-05-07 16:25 ` Steven Rostedt 2019-05-07 15:31 ` torvalds 2019-05-07 15:31 ` Linus Torvalds 2019-05-07 15:45 ` rostedt 2019-05-07 15:45 ` Steven Rostedt 2019-05-07 16:34 ` peterz 2019-05-07 16:34 ` Peter Zijlstra 2019-05-07 17:08 ` torvalds 2019-05-07 17:08 ` Linus Torvalds 2019-05-07 17:21 ` jpoimboe 2019-05-07 17:21 ` Josh Poimboeuf 2019-05-07 21:24 ` rostedt 2019-05-07 21:24 ` Steven Rostedt 2019-05-08 4:50 ` torvalds 2019-05-08 4:50 ` Linus Torvalds 2019-05-08 16:37 ` rostedt 2019-05-08 16:37 ` Steven Rostedt 2019-05-07 17:38 ` peterz 2019-05-07 17:38 ` Peter Zijlstra 2019-05-07 9:51 ` peterz 2019-05-07 9:51 ` Peter Zijlstra 2019-05-07 14:48 ` luto 2019-05-07 14:48 ` Andy Lutomirski 2019-05-07 14:57 ` torvalds 2019-05-07 14:57 ` Linus Torvalds 2019-05-07 14:13 ` mhiramat 2019-05-07 14:13 ` Masami Hiramatsu 2019-05-07 17:15 ` mhiramat 2019-05-07 17:15 ` Masami Hiramatsu 2019-05-06 14:22 ` peterz 2019-05-06 14:22 ` Peter Zijlstra 2019-05-07 8:57 ` peterz 2019-05-07 8:57 ` Peter Zijlstra 2019-05-07 9:18 ` David.Laight 2019-05-07 9:18 ` David Laight 2019-05-07 11:30 ` peterz 2019-05-07 11:30 ` Peter Zijlstra 2019-05-07 12:57 ` David.Laight 2019-05-07 12:57 ` David Laight 2019-05-07 13:14 ` rostedt 2019-05-07 13:14 ` Steven Rostedt 2019-05-07 14:50 ` David.Laight 2019-05-07 14:50 ` David Laight 2019-05-07 14:57 ` rostedt [this message] 2019-05-07 14:57 ` Steven Rostedt 2019-05-07 15:46 ` David.Laight 2019-05-07 15:46 ` David Laight 2019-05-07 13:32 ` peterz 2019-05-07 13:32 ` Peter Zijlstra 2019-05-07 9:27 ` peterz 2019-05-07 9:27 ` Peter Zijlstra 2019-05-07 12:27 ` rostedt 2019-05-07 12:27 ` Steven Rostedt 2019-05-07 12:41 ` peterz 2019-05-07 12:41 ` Peter Zijlstra 2019-05-07 12:54 ` rostedt 2019-05-07 12:54 ` Steven Rostedt 2019-05-07 17:22 ` masami.hiramatsu 2019-05-07 17:22 ` Masami Hiramatsu 2019-05-07 14:28 ` peterz 2019-05-07 14:28 ` Peter Zijlstra 2019-05-02 20:48 ` rostedt 2019-05-02 20:48 ` Steven Rostedt 2019-05-06 15:14 ` jpoimboe 2019-05-06 15:14 ` Josh Poimboeuf 2019-05-01 20:28 ` [RFC][PATCH 2/2] ftrace/x86: Emulate call function while updating in breakpoint handler rostedt 2019-05-01 20:28 ` Steven Rostedt 2019-05-03 10:22 ` [RFC][PATCH 1.5/2] x86: Add int3_emulate_call() selftest peterz 2019-05-03 10:22 ` Peter Zijlstra 2019-05-03 18:46 ` rostedt 2019-05-03 18:46 ` Steven Rostedt
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20190507105724.02abe6f6@gandalf.local.home \ --to=linux-kselftest@vger.kernel.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).