Thursday, November 17, 2011

Your own linker warnings using the GNU toolchain

You've probably seen linker warnings like these:

linkwarnmain.c:(.text+0x1d): warning: foo is deprecated, please use the shiny new foobar function
linkwarnmain.c:(.text+0x27): warning: the use of `tmpnam' is dangerous, better use `mkstemp'

These warnings are actually stored in ELF sections named .gnu.warning.symbolname:

$ objdump -s -j .gnu.warning.gets /lib/

/lib/     file format elf32-i386

Contents of section .gnu.warning.gets:
 0000 74686520 60676574 73272066 756e6374  the `gets' funct
 0010 696f6e20 69732064 616e6765 726f7573  ion is dangerous
 0020 20616e64 2073686f 756c6420 6e6f7420   and should not
 0030 62652075 7365642e 00                 be used..

So when you try to use a symbol, if the linker sees a section whose name matches the above pattern, it emits the corresponding warning message.

You can add your own linker warnings to your source files:

void foo(void)

static const char foo_warning[] __attribute__((section(""))) =
        "foo is deprecated, please use the shiny new foobar function";

void foo(void);

int main()

        return 0;

Then, when you compile, you will get a warning:

$ gcc -Wall -c linkwarn.c
$ gcc -Wall -o linkwarnmain linkwarnmain.c linkwarn.o
/tmp/ccyHLTw6.o: In function `main':
linkwarnmain.c:(.text+0x1d): warning: foo is deprecated, please use the shiny new foobar function

glibc machinery for emitting linker warnings is a little bit more complicated:

#ifdef HAVE_ELF

/* We want the .gnu.warning.SYMBOL section to be unallocated.  */
#  define __make_section_unallocated(section_string)    \
  asm (".section " section_string "\n\t.previous");
#  define __make_section_unallocated(section_string)    \
  asm (".pushsection " section_string "\n\t.popsection");
# else
#  define __make_section_unallocated(section_string)
# endif

/* Tacking on "\n\t#" to the section name makes gcc put it's bogus
   section attributes on what looks like a comment to the assembler.  */
#  define __sec_comment "\"\n\t#\""
# else
#  define __sec_comment "\n\t#"
# endif
# define link_warning(symbol, msg) \
  __make_section_unallocated (".gnu.warning." #symbol) \
  static const char __evoke_link_warning_##symbol[]     \
    __attribute__ ((used, section (".gnu.warning." #symbol __sec_comment))) \
    = msg;

The warning message is marked as used so the optimizer doesn't decide to completely optimize it out of existence. Also, the section is not marked as allocatable to prevent the loader from loading it into memory. Since gcc's section attribute doesn't allow to change its default flags, the solution is to declare the section using asm, return to the previous section (the one the compiler was using beforehand), and prevent (via __sec_comment) the assembler from seeing the flags that gcc adds in its section attribute output. Otherwise we'd have something similar to:

$ gcc -Wall -S linkwarn.c
$ cat linkwarn.s
        .file   "linkwarn.c"
.globl foo
        .type   foo, @function
        pushl   %ebp
        movl    %esp, %ebp
        popl    %ebp
        .size   foo, .-foo
        .align 32
        .type   foo_warning, @object
        .size   foo_warning, 60
        .string "foo is deprecated, please use the shiny new foobar function"
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.5 (Gentoo 3.4.5-r1, ssp-3.4.5-1.0, pie-8.7.9)"

... where the section is marked as allocatable via the "a" flag.