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    Subject[RT:PATCH] tracing/x86: Add check to GCC messing with mcount prologue

    This patch is originally from Steven Rostedt.
    I cherry-picked it from his git repo, and ported it to v2.6.33.2-rt13
    by fixing up the merge conflicts.

    The immediate importance of this, is that it restores the function_graph
    tracer to 32-bit x86, for real-time.

    I have done some light testing on 32-bit x86, but it could use some more
    testing everywhere. After that I hope that Thomas will pick it up for the
    next 2.6.33.2-rtN release.

    I have done this for real-time purposes, but Steven's patch was intended
    for the mainline kernel, so maybe some testing in -rt would convince
    folks to pick it up for mainline as well, if it isn't already queued up.

    John Kacur


    From 8f31a8a1cc4f1ee54ad673318954b118e55aca31 Mon Sep 17 00:00:00 2001
    From: Steven Rostedt <srostedt@redhat.com>
    Date: Thu, 19 Nov 2009 23:41:02 -0500
    Subject: [PATCH] tracing/x86: Add check to detect GCC messing with mcount prologue

    Latest versions of GCC create a funny prologue for some functions.
    Instead of the typical:

    push %ebp
    mov %esp,%ebp
    and $0xffffffe0,%esp
    [...]
    call mcount

    GCC may try to align the stack before setting up the frame pointer
    register:

    push %edi
    lea 0x8(%esp),%edi
    and $0xffffffe0,%esp
    pushl -0x4(%edi)
    push %ebp
    mov %esp,%ebp
    [...]
    call mcount

    This crazy code places a copy of the return address into the
    frame pointer. The function graph tracer uses this pointer to
    save and replace the return address of the calling function to jump
    to the function graph tracer's return handler, which will put back
    the return address. But instead instead of the typical return:

    mov %ebp,%esp
    pop %ebp
    ret

    The return of the function performs:

    lea -0x8(%edi),%esp
    pop %edi
    ret

    The function graph tracer return handler will not be called at the exit
    of the function, but the parent function will call it. Because we missed
    the return of the child function, the handler will replace the parent's
    return address with that of the child. Obviously this will cause a crash
    (Note, there is code to detect this case and safely panic the kernel).

    The kicker is that this happens to just a handful of functions.
    And only with certain gcc options.

    Compiling with: -march=pentium-mmx
    will cause the problem to appear. But if you were to change
    pentium-mmx to i686 or add -mtune=generic, then the problem goes away.

    I first saw this problem when compiling with optimize for size.
    But it seems that various other options may cause this issue to arise.

    Instead of completely disabling the function graph tracer for i386 builds
    this patch adds a check to recordmcount.pl to make sure that all
    functions that contain a call to mcount start with "push %ebp".
    If not, it will fail the compile and print out the nasty warning:

    CC kernel/time/timer_stats.o

    ********************************************************
    Your version of GCC breaks the function graph tracer
    Please disable CONFIG_FUNCTION_GRAPH_TRACER
    Failed function was "timer_stats_update_stats"
    ********************************************************

    The script recordmcount.pl is given a new parameter "do_check". If
    this is negative, the script will only perform this check without
    creating the mcount caller section. This will be executed for x86_32
    when CONFIG_FUNCTION_GRAPH_TRACER is enabled and CONFIG_DYNAMIC_FTRACE
    is not.

    If the arch is x86_32 and $do_check is greater than 1, it will perform
    the check while processing the mcount callers. If $do_check is 0, then
    no check will be performed. This is for non x86_32 archs and when
    compiling without CONFIG_FUNCTION_GRAPH_TRACER enabled, even on x86_32.

    Reported-by: Thomas Gleixner <tglx@linutronix.de>
    LKML-Reference: <alpine.LFD.2.00.0911191423190.24119@localhost.localdomain>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

    Fixed merge conflicts when cherry-picking this for v2.6.33.2-rt13
    Signed-off-by: John Kacur <jkacur@redhat.com>
    ---
    kernel/trace/Kconfig | 1 -
    scripts/Makefile.build | 25 +++++++++++++++-
    scripts/recordmcount.pl | 72 +++++++++++++++++++++++++++++++++++++++++++++--
    3 files changed, 93 insertions(+), 5 deletions(-)

    diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
    index 7d57890..159ba62 100644
    --- a/kernel/trace/Kconfig
    +++ b/kernel/trace/Kconfig
    @@ -141,7 +141,6 @@ config FUNCTION_GRAPH_TRACER
    bool "Kernel Function Graph Tracer"
    depends on HAVE_FUNCTION_GRAPH_TRACER
    depends on FUNCTION_TRACER
    - depends on !X86_32 || !CC_OPTIMIZE_FOR_SIZE
    default y
    help
    Enable the kernel to trace a function at both its return
    diff --git a/scripts/Makefile.build b/scripts/Makefile.build
    index 0b94d2f..6dfdc91 100644
    --- a/scripts/Makefile.build
    +++ b/scripts/Makefile.build
    @@ -206,11 +206,34 @@ cmd_modversions = \
    endif

    ifdef CONFIG_FTRACE_MCOUNT_RECORD
    +
    + ifdef CONFIG_FUNCTION_GRAPH_TRACER
    + ifdef CONFIG_X86_32
    + rm_do_check = 1
    + else
    + rm_do_check = 0
    + endif
    + else
    + rm_do_check = 0
    + endif
    +
    cmd_record_mcount = set -e ; perl $(srctree)/scripts/recordmcount.pl "$(ARCH)" \
    "$(if $(CONFIG_CPU_BIG_ENDIAN),big,little)" \
    "$(if $(CONFIG_64BIT),64,32)" \
    "$(OBJDUMP)" "$(OBJCOPY)" "$(CC)" "$(LD)" "$(NM)" "$(RM)" "$(MV)" \
    - "$(if $(part-of-module),1,0)" "$(@)";
    + "$(if $(part-of-module),1,0)" "$(rm_do_check)" "$(@)";
    +
    +else
    +
    + ifdef CONFIG_X86_32
    + ifdef CONFIG_FUNCTION_GRAPH_TRACER
    + cmd_record_mcount = set -e ; perl $(srctree)/scripts/recordmcount.pl "$(ARCH)" \
    + "$(if $(CONFIG_64BIT),64,32)" \
    + "$(OBJDUMP)" "$(OBJCOPY)" "$(CC)" "$(LD)" "$(NM)" "$(RM)" "$(MV)" \
    + "$(if $(part-of-module),1,0)" "-1" "$(@)";
    + endif
    + endif
    +
    endif

    define rule_cc_o_c
    diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
    index ea6f6e3..22c4644 100755
    --- a/scripts/recordmcount.pl
    +++ b/scripts/recordmcount.pl
    @@ -113,14 +113,14 @@ $P =~ s@.*/@@g;

    my $V = '0.1';

    -if ($#ARGV != 11) {
    - print "usage: $P arch endian bits objdump objcopy cc ld nm rm mv is_module inputfile\n";
    +if ($#ARGV < 12) {
    + print "usage: $P arch endian bits objdump objcopy cc ld nm rm mv is_module do_check inputfile\n";
    print "version: $V\n";
    exit(1);
    }

    my ($arch, $endian, $bits, $objdump, $objcopy, $cc,
    - $ld, $nm, $rm, $mv, $is_module, $inputfile) = @ARGV;
    + $ld, $nm, $rm, $mv, $is_module, $do_check, $inputfile) = @ARGV;

    # This file refers to mcount and shouldn't be ftraced, so lets' ignore it
    if ($inputfile =~ m,kernel/trace/ftrace\.o$,) {
    @@ -144,6 +144,60 @@ $nm = "nm" if ((length $nm) == 0);
    $rm = "rm" if ((length $rm) == 0);
    $mv = "mv" if ((length $mv) == 0);

    +# gcc can do stupid things with the stack pointer on x86_32.
    +# It may pass a copy of the return address to mcount, which will
    +# break the function graph tracer. If this happens then we need
    +# to flag it and break the build.
    +#
    +# For x86_32, the parameter do_check will be negative if we only
    +# want to perform the check, and positive if we should od the check.
    +# If function graph tracing is not enabled, do_check will be zero.
    +#
    +
    +my $check_next_line = 0;
    +my $line_failed = 0;
    +my $last_function;
    +
    +sub test_x86_32_prologue
    +{
    + if ($check_next_line) {
    + if (!/push\s*\%ebp/) {
    + $line_failed = 1;
    + }
    + }
    +
    + if ($line_failed && /mcount/) {
    + print STDERR "\n********************************************************\n";
    + print STDERR " Your version of GCC breaks the function graph tracer\n";
    + print STDERR " Please disable CONFIG_FUNCTION_GRAPH_TRACER\n";
    + print STDERR " Failed function was \"$last_function\"\n";
    + print STDERR "********************************************************\n\n";
    + exit -1;
    + }
    + $check_next_line = 0;
    +
    + # check the first line after a function starts for
    + # push %ebp
    + if (/^[0-9a-fA-F]+\s+<([a-zA-Z_].*)>:$/) {
    + $last_function = $1;
    + $check_next_line = 1;
    + $line_failed = 0;
    + }
    +}
    +
    +if ($do_check < 0) {
    + # Only perform the check and quit
    + open(IN, "$objdump -dr $inputfile|") || die "error running $objdump";
    +
    + while (<IN>) {
    + test_x86_32_prologue;
    + }
    + close (IN);
    + exit 0;
    +}
    +
    +my $check = 0;
    +
    #print STDERR "running: $P '$arch' '$objdump' '$objcopy' '$cc' '$ld' " .
    # "'$nm' '$rm' '$mv' '$inputfile'\n";

    @@ -202,6 +256,12 @@ if ($arch =~ /(x86(_64)?)|(i386)/) {
    }
    }

    +if ($arch eq "i386") {
    + if ($do_check) {
    + $check = 1;
    + }
    +}
    +
    #
    # We base the defaults off of i386, the other archs may
    # feel free to change them in the below if statements.
    @@ -470,6 +530,12 @@ while (<IN>) {
    "\tstill causes an issue, then disable CONFIG_DYNAMIC_FTRACE.\n";
    exit(-1);
    }
    + # x86_32 may need to test the start of every function to see
    + # if GCC did not mess up the mcount prologue. All functions must
    + # start with push %ebp, otherwise it is broken.
    + if ($check) {
    + test_x86_32_prologue;
    + }

    # is it a section?
    if (/$section_regex/) {
    --
    1.6.6.1


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