• Josh Poimboeuf's avatar
    objtool: Add tool to perform compile-time stack metadata validation · 442f04c3
    Josh Poimboeuf authored
    This adds a host tool named objtool which has a "check" subcommand which
    analyzes .o files to ensure the validity of stack metadata.  It enforces
    a set of rules on asm code and C inline assembly code so that stack
    traces can be reliable.
    For each function, it recursively follows all possible code paths and
    validates the correct frame pointer state at each instruction.
    It also follows code paths involving kernel special sections, like
    .altinstructions, __jump_table, and __ex_table, which can add
    alternative execution paths to a given instruction (or set of
    instructions).  Similarly, it knows how to follow switch statements, for
    which gcc sometimes uses jump tables.
    Here are some of the benefits of validating stack metadata:
    a) More reliable stack traces for frame pointer enabled kernels
       Frame pointers are used for debugging purposes.  They allow runtime
       code and debug tools to be able to walk the stack to determine the
       chain of function call sites that led to the currently executing
       For some architectures, frame pointers are enabled by
       CONFIG_FRAME_POINTER.  For some other architectures they may be
       required by the ABI (sometimes referred to as "backchain pointers").
       For C code, gcc automatically generates instructions for setting up
       frame pointers when the -fno-omit-frame-pointer option is used.
       But for asm code, the frame setup instructions have to be written by
       hand, which most people don't do.  So the end result is that
       CONFIG_FRAME_POINTER is honored for C code but not for most asm code.
       For stack traces based on frame pointers to be reliable, all
       functions which call other functions must first create a stack frame
       and update the frame pointer.  If a first function doesn't properly
       create a stack frame before calling a second function, the *caller*
       of the first function will be skipped on the stack trace.
       For example, consider the following example backtrace with frame
       pointers enabled:
         [<ffffffff81812584>] dump_stack+0x4b/0x63
         [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
         [<ffffffff8127f568>] seq_read+0x108/0x3e0
         [<ffffffff812cce62>] proc_reg_read+0x42/0x70
         [<ffffffff81256197>] __vfs_read+0x37/0x100
         [<ffffffff81256b16>] vfs_read+0x86/0x130
         [<ffffffff81257898>] SyS_read+0x58/0xd0
         [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
       It correctly shows that the caller of cmdline_proc_show() is
       If we remove the frame pointer logic from cmdline_proc_show() by
       replacing the frame pointer related instructions with nops, here's
       what it looks like instead:
         [<ffffffff81812584>] dump_stack+0x4b/0x63
         [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
         [<ffffffff812cce62>] proc_reg_read+0x42/0x70
         [<ffffffff81256197>] __vfs_read+0x37/0x100
         [<ffffffff81256b16>] vfs_read+0x86/0x130
         [<ffffffff81257898>] SyS_read+0x58/0xd0
         [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
       Notice that cmdline_proc_show()'s caller, seq_read(), has been
       skipped.  Instead the stack trace seems to show that
       cmdline_proc_show() was called by proc_reg_read().
       The benefit of "objtool check" here is that because it ensures that
       *all* functions honor CONFIG_FRAME_POINTER, no functions will ever[*]
       be skipped on a stack trace.
       [*] unless an interrupt or exception has occurred at the very
           beginning of a function before the stack frame has been created,
           or at the very end of the function after the stack frame has been
           destroyed.  This is an inherent limitation of frame pointers.
    b) 100% reliable stack traces for DWARF enabled kernels
       This is not yet implemented.  For more details about what is planned,
       see tools/objtool/Documentation/stack-validation.txt.
    c) Higher live patching compatibility rate
       This is not yet implemented.  For more details about what is planned,
       see tools/objtool/Documentation/stack-validation.txt.
    To achieve the validation, "objtool check" enforces the following rules:
    1. Each callable function must be annotated as such with the ELF
       function type.  In asm code, this is typically done using the
       ENTRY/ENDPROC macros.  If objtool finds a return instruction
       outside of a function, it flags an error since that usually indicates
       callable code which should be annotated accordingly.
       This rule is needed so that objtool can properly identify each
       callable function in order to analyze its stack metadata.
    2. Conversely, each section of code which is *not* callable should *not*
       be annotated as an ELF function.  The ENDPROC macro shouldn't be used
       in this case.
       This rule is needed so that objtool can ignore non-callable code.
       Such code doesn't have to follow any of the other rules.
    3. Each callable function which calls another function must have the
       correct frame pointer logic, if required by CONFIG_FRAME_POINTER or
       the architecture's back chain rules.  This can by done in asm code
       with the FRAME_BEGIN/FRAME_END macros.
       This rule ensures that frame pointer based stack traces will work as
       designed.  If function A doesn't create a stack frame before calling
       function B, the _caller_ of function A will be skipped on the stack
    4. Dynamic jumps and jumps to undefined symbols are only allowed if:
       a) the jump is part of a switch statement; or
       b) the jump matches sibling call semantics and the frame pointer has
          the same value it had on function entry.
       This rule is needed so that objtool can reliably analyze all of a
       function's code paths.  If a function jumps to code in another file,
       and it's not a sibling call, objtool has no way to follow the jump
       because it only analyzes a single file at a time.
    5. A callable function may not execute kernel entry/exit instructions.
       The only code which needs such instructions is kernel entry code,
       which shouldn't be be in callable functions anyway.
       This rule is just a sanity check to ensure that callable functions
       return normally.
    It currently only supports x86_64.  I tried to make the code generic so
    that support for other architectures can hopefully be plugged in
    relatively easily.
    On my Lenovo laptop with a i7-4810MQ 4-core/8-thread CPU, building the
    kernel with objtool checking every .o file adds about three seconds of
    total build time.  It hasn't been optimized for performance yet, so
    there are probably some opportunities for better build performance.
    Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
    Cc: Bernd Petrovitsch <bernd@petrovitsch.priv.at>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Chris J Arges <chris.j.arges@canonical.com>
    Cc: Jiri Slaby <jslaby@suse.cz>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Michal Marek <mmarek@suse.cz>
    Cc: Namhyung Kim <namhyung@gmail.com>
    Cc: Pedro Alves <palves@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: live-patching@vger.kernel.org
    Link: http://lkml.kernel.org/r/f3efb173de43bd067b060de73f856567c0fa1174.1456719558.git.jpoimboe@redhat.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
special.c 4.57 KB