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    SubjectRe: [PATCH] kernel: Fix ftrace.c compiler warning when calling ftrace_nop_replace()


    On Mon, 23 May 2011, Steven Rostedt wrote:

    > On Sun, 2011-05-22 at 18:36 -0700, H. Peter Anvin wrote:
    > > On 05/22/2011 06:19 PM, Eduardo Silva wrote:
    > > > the function ftrace_nop_replace() returns a 'static const unsigned char *'
    > > > value, so when the caller perform a direct assignment to a
    > > > 'static unsigned char *', the compiler raise the following warning message:
    > > >
    > > > ftrace.c:308:6: warning: assignment discards qualifiers from pointer target type
    > > > ftrace.c:318:6: warning: assignment discards qualifiers from pointer target type
    > > >
    > > > Adding the proper casts the message goes away.
    > > >
    > > > Signed-off-by: Eduardo Silva <edsiper@gmail.com>
    > >
    > > This quiets a warning of something that potentially looks like a real
    > > bug. Perhaps "new" should be const?
    >
    > Although there is no real bug here, I hate these "hide the warning"
    > patches.
    >
    > The real solution is to change all the users into const *. This should
    > be doable as the new and old pointers are not modified.
    >
    > And remember:
    >
    > char *changes;
    > const char *no_changes;
    >
    > no_changes = changes; /* is OK! (no compiler warnings) */
    >
    > changes = no_changes; /* is not OK. (compiler will warn). */
    >
    > -- Steve

    The following needs some testing, just compile tested for now.

    From 71d6e5971cc90a03fd2d7bf8aaa2b81932bd0c7c Mon Sep 17 00:00:00 2001
    From: John Kacur <jkacur@redhat.com>
    Date: Mon, 23 May 2011 21:24:27 +0200
    Subject: [PATCH] ftrace: Change unsigned char *new, *old to const
    MIME-Version: 1.0
    Content-Type: text/plain; charset=UTF-8
    Content-Transfer-Encoding: 8bit

    This removes the warning:
    arch/x86/kernel/ftrace.c: In function ‘ftrace_make_nop’:
    /arch/x86/kernel/ftrace.c:308: warning: assignment discards qualifiers from pointer target type

    Steven Rostedt suggested to make these pointers const pointers.

    This patch assumes that
    https://patchwork.kernel.org/patch/798172/
    is applied first.

    Reported-by: Eduardo Silva <edsiper@gmail.com>
    Signed-off-by: John Kacur <jkacur@redhat.com>
    ---
    arch/x86/kernel/ftrace.c | 12 ++++++------
    1 files changed, 6 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
    index 0ba15a6..e4f23d1 100644
    --- a/arch/x86/kernel/ftrace.c
    +++ b/arch/x86/kernel/ftrace.c
    @@ -123,7 +123,7 @@ static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
    static atomic_t nmi_running = ATOMIC_INIT(0);
    static int mod_code_status; /* holds return value of text write */
    static void *mod_code_ip; /* holds the IP to write to */
    -static void *mod_code_newcode; /* holds the text to write to the IP */
    +static const void *mod_code_newcode; /* holds the text to write to the IP */

    static unsigned nmi_wait_count;
    static atomic_t nmi_update_count = ATOMIC_INIT(0);
    @@ -225,7 +225,7 @@ within(unsigned long addr, unsigned long start, unsigned long end)
    }

    static int
    -do_ftrace_mod_code(unsigned long ip, void *new_code)
    +do_ftrace_mod_code(unsigned long ip, const void *new_code)
    {
    /*
    * On x86_64, kernel text mappings are mapped read-only with
    @@ -266,8 +266,8 @@ static const unsigned char *ftrace_nop_replace(void)
    }

    static int
    -ftrace_modify_code(unsigned long ip, unsigned char *old_code,
    - unsigned char *new_code)
    +ftrace_modify_code(unsigned long ip, const unsigned char *old_code,
    + const unsigned char *new_code)
    {
    unsigned char replaced[MCOUNT_INSN_SIZE];

    @@ -301,7 +301,7 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
    int ftrace_make_nop(struct module *mod,
    struct dyn_ftrace *rec, unsigned long addr)
    {
    - unsigned char *new, *old;
    + const unsigned char *new, *old;
    unsigned long ip = rec->ip;

    old = ftrace_call_replace(ip, addr);
    @@ -312,7 +312,7 @@ int ftrace_make_nop(struct module *mod,

    int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
    {
    - unsigned char *new, *old;
    + const unsigned char *new, *old;
    unsigned long ip = rec->ip;

    old = ftrace_nop_replace();
    --
    1.7.2.3
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