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    Subject[PATCH 4.4 054/149] tracepoint: Mark __tracepoint_strings __used
    Date
    From: Nick Desaulniers <ndesaulniers@google.com>

    commit f3751ad0116fb6881f2c3c957d66a9327f69cefb upstream.

    __tracepoint_string's have their string data stored in .rodata, and an
    address to that data stored in the "__tracepoint_str" section. Functions
    that refer to those strings refer to the symbol of the address. Compiler
    optimization can replace those address references with references
    directly to the string data. If the address doesn't appear to have other
    uses, then it appears dead to the compiler and is removed. This can
    break the /tracing/printk_formats sysfs node which iterates the
    addresses stored in the "__tracepoint_str" section.

    Like other strings stored in custom sections in this header, mark these
    __used to inform the compiler that there are other non-obvious users of
    the address, so they should still be emitted.

    Link: https://lkml.kernel.org/r/20200730224555.2142154-2-ndesaulniers@google.com

    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
    Cc: stable@vger.kernel.org
    Fixes: 102c9323c35a8 ("tracing: Add __tracepoint_string() to export string pointers")
    Reported-by: Tim Murray <timmurray@google.com>
    Reported-by: Simon MacMullen <simonmacm@google.com>
    Suggested-by: Greg Hackmann <ghackmann@google.com>
    Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/tracepoint.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/include/linux/tracepoint.h
    +++ b/include/linux/tracepoint.h
    @@ -328,7 +328,7 @@ extern void syscall_unregfunc(void);
    static const char *___tp_str __tracepoint_string = str; \
    ___tp_str; \
    })
    -#define __tracepoint_string __attribute__((section("__tracepoint_str")))
    +#define __tracepoint_string __attribute__((section("__tracepoint_str"), used))
    #else
    /*
    * tracepoint_string() is used to save the string address for userspace

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