objtool: Implement stack validation 2.0

This is a major rewrite of objtool.  Instead of only tracking frame
pointer changes, it now tracks all stack-related operations, including
all register saves/restores.

In addition to making stack validation more robust, this also paves the
way for undwarf generation.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: live-patching@vger.kernel.org
Link: http://lkml.kernel.org/r/678bd94c0566c6129bcc376cddb259c4c5633004.1498659915.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 65 insertions, 88 deletions

diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index 55a60d331f47..17c1195f11f4 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/tools/objtool/Documentation/stack-validation.txt
@@ -127,28 +127,13 @@ b) 100% reliable stack traces for DWARF enabled kernels
 
 c) Higher live patching compatibility rate
 
-   (NOTE: This is not yet implemented)
-
-   Currently with CONFIG_LIVEPATCH there's a basic live patching
-   framework which is safe for roughly 85-90% of "security" fixes.  But
-   patches can't have complex features like function dependency or
-   prototype changes, or data structure changes.
-
-   There's a strong need to support patches which have the more complex
-   features so that the patch compatibility rate for security fixes can
-   eventually approach something resembling 100%.  To achieve that, a
-   "consistency model" is needed, which allows tasks to be safely
-   transitioned from an unpatched state to a patched state.
-
-   One of the key requirements of the currently proposed livepatch
-   consistency model [*] is that it needs to walk the stack of each
-   sleeping task to determine if it can be transitioned to the patched
-   state.  If objtool can ensure that stack traces are reliable, this
-   consistency model can be used and the live patching compatibility
-   rate can be improved significantly.
-
-   [*] https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@redhat.com
+   Livepatch has an optional "consistency model", which is needed for
+   more complex patches.  In order for the consistency model to work,
+   stack traces need to be reliable (or an unreliable condition needs to
+   be detectable).  Objtool makes that possible.
 
+   For more details, see the livepatch documentation in the Linux kernel
+   source tree at Documentation/livepatch/livepatch.txt.
 
 Rules
 -----
@@ -201,80 +186,84 @@ To achieve the validation, objtool enforces the following rules:
    return normally.
 
 
-Errors in .S files
-------------------
+Objtool warnings
+----------------
 
-If you're getting an error in a compiled .S file which you don't
-understand, first make sure that the affected code follows the above
-rules.
+For asm files, if you're getting an error which doesn't make sense,
+first make sure that the affected code follows the above rules.
+
+For C files, the common culprits are inline asm statements and calls to
+"noreturn" functions.  See below for more details.
+
+Another possible cause for errors in C code is if the Makefile removes
+-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.
 
 Here are some examples of common warnings reported by objtool, what
 they mean, and suggestions for how to fix them.
 
 
-1. asm_file.o: warning: objtool: func()+0x128: call without frame pointer save/setup
+1. file.o: warning: objtool: func()+0x128: call without frame pointer save/setup
 
    The func() function made a function call without first saving and/or
-   updating the frame pointer.
-
-   If func() is indeed a callable function, add proper frame pointer
-   logic using the FRAME_BEGIN and FRAME_END macros.  Otherwise, remove
-   its ELF function annotation by changing ENDPROC to END.
+   updating the frame pointer, and CONFIG_FRAME_POINTER is enabled.
 
-   If you're getting this error in a .c file, see the "Errors in .c
-   files" section.
+   If the error is for an asm file, and func() is indeed a callable
+   function, add proper frame pointer logic using the FRAME_BEGIN and
+   FRAME_END macros.  Otherwise, if it's not a callable function, remove
+   its ELF function annotation by changing ENDPROC to END, and instead
+   use the manual CFI hint macros in asm/undwarf.h.
 
+   If it's a GCC-compiled .c file, the error may be because the function
+   uses an inline asm() statement which has a "call" instruction.  An
+   asm() statement with a call instruction must declare the use of the
+   stack pointer in its output operand.  For example, on x86_64:
 
-2. asm_file.o: warning: objtool: .text+0x53: return instruction outside of a callable function
-
-   A return instruction was detected, but objtool couldn't find a way
-   for a callable function to reach the instruction.
+     register void *__sp asm("rsp");
+     asm volatile("call func" : "+r" (__sp));
 
-   If the return instruction is inside (or reachable from) a callable
-   function, the function needs to be annotated with the ENTRY/ENDPROC
-   macros.
+   Otherwise the stack frame may not get created before the call.
 
-   If you _really_ need a return instruction outside of a function, and
-   are 100% sure that it won't affect stack traces, you can tell
-   objtool to ignore it.  See the "Adding exceptions" section below.
 
+2. file.o: warning: objtool: .text+0x53: unreachable instruction
 
-3. asm_file.o: warning: objtool: func()+0x9: function has unreachable instruction
+   Objtool couldn't find a code path to reach the instruction.
 
-   The instruction lives inside of a callable function, but there's no
-   possible control flow path from the beginning of the function to the
-   instruction.
+   If the error is for an asm file, and the instruction is inside (or
+   reachable from) a callable function, the function should be annotated
+   with the ENTRY/ENDPROC macros (ENDPROC is the important one).
+   Otherwise, the code should probably be annotated with the CFI hint
+   macros in asm/undwarf.h so objtool and the unwinder can know the
+   stack state associated with the code.
 
-   If the instruction is actually needed, and it's actually in a
-   callable function, ensure that its function is properly annotated
-   with ENTRY/ENDPROC.
+   If you're 100% sure the code won't affect stack traces, or if you're
+   a just a bad person, you can tell objtool to ignore it.  See the
+   "Adding exceptions" section below.
 
    If it's not actually in a callable function (e.g. kernel entry code),
    change ENDPROC to END.
 
 
-4. asm_file.o: warning: objtool: func(): can't find starting instruction
+4. file.o: warning: objtool: func(): can't find starting instruction
    or
-   asm_file.o: warning: objtool: func()+0x11dd: can't decode instruction
+   file.o: warning: objtool: func()+0x11dd: can't decode instruction
 
-   Did you put data in a text section?  If so, that can confuse
+   Does the file have data in a text section?  If so, that can confuse
    objtool's instruction decoder.  Move the data to a more appropriate
    section like .data or .rodata.
 
 
-5. asm_file.o: warning: objtool: func()+0x6: kernel entry/exit from callable instruction
-
-   This is a kernel entry/exit instruction like sysenter or sysret.
-   Such instructions aren't allowed in a callable function, and are most
-   likely part of the kernel entry code.
+5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function
 
-   If the instruction isn't actually in a callable function, change
-   ENDPROC to END.
+   This is a kernel entry/exit instruction like sysenter or iret.  Such
+   instructions aren't allowed in a callable function, and are most
+   likely part of the kernel entry code.  They should usually not have
+   the callable function annotation (ENDPROC) and should always be
+   annotated with the CFI hint macros in asm/undwarf.h.
 
 
-6. asm_file.o: warning: objtool: func()+0x26: sibling call from callable instruction with changed frame pointer
+6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame
 
-   This is a dynamic jump or a jump to an undefined symbol.  Stacktool
+   This is a dynamic jump or a jump to an undefined symbol.  Objtool
    assumed it's a sibling call and detected that the frame pointer
    wasn't first restored to its original state.
 
@@ -282,24 +271,28 @@ they mean, and suggestions for how to fix them.
    destination code to the local file.
 
    If the instruction is not actually in a callable function (e.g.
-   kernel entry code), change ENDPROC to END.
+   kernel entry code), change ENDPROC to END and annotate manually with
+   the CFI hint macros in asm/undwarf.h.
 
 
-7. asm_file: warning: objtool: func()+0x5c: frame pointer state mismatch
+7. file: warning: objtool: func()+0x5c: stack state mismatch
 
    The instruction's frame pointer state is inconsistent, depending on
    which execution path was taken to reach the instruction.
 
-   Make sure the function pushes and sets up the frame pointer (for
-   x86_64, this means rbp) at the beginning of the function and pops it
-   at the end of the function.  Also make sure that no other code in the
-   function touches the frame pointer.
+   Make sure that, when CONFIG_FRAME_POINTER is enabled, the function
+   pushes and sets up the frame pointer (for x86_64, this means rbp) at
+   the beginning of the function and pops it at the end of the function.
+   Also make sure that no other code in the function touches the frame
+   pointer.
 
+   Another possibility is that the code has some asm or inline asm which
+   does some unusual things to the stack or the frame pointer.  In such
+   cases it's probably appropriate to use the CFI hint macros in
+   asm/undwarf.h.
 
-Errors in .c files
-------------------
 
-1. c_file.o: warning: objtool: funcA() falls through to next function funcB()
+8. file.o: warning: objtool: funcA() falls through to next function funcB()
 
    This means that funcA() doesn't end with a return instruction or an
    unconditional jump, and that objtool has determined that the function
@@ -318,22 +311,6 @@ Errors in .c files
       might be corrupt due to a gcc bug.  For more details, see:
       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
 
-2. If you're getting any other objtool error in a compiled .c file, it
-   may be because the file uses an asm() statement which has a "call"
-   instruction.  An asm() statement with a call instruction must declare
-   the use of the stack pointer in its output operand.  For example, on
-   x86_64:
-
-     register void *__sp asm("rsp");
-     asm volatile("call func" : "+r" (__sp));
-
-   Otherwise the stack frame may not get created before the call.
-
-3. Another possible cause for errors in C code is if the Makefile removes
-   -fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.
-
-Also see the above section for .S file errors for more information what
-the individual error messages mean.
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask the objtool maintainer for help.