• Willy Tarreau's avatar
    tools/nolibc: Fix missing strlen() definition and infinite loop with gcc-12 · bfc3b0f0
    Willy Tarreau authored
    When built at -Os, gcc-12 recognizes an strlen() pattern in nolibc_strlen()
    and replaces it with a jump to strlen(), which is not defined as a symbol
    and breaks compilation. Worse, when the function is called strlen(), the
    function is simply replaced with a jump to itself, hence becomes an
    infinite loop.
    
    One way to avoid this is to always set -ffreestanding, but the calling
    code doesn't know this and there's no way (either via attributes or
    pragmas) to globally enable it from include files, effectively leaving
    a painful situation for the caller.
    
    Alexey suggested to place an empty asm() statement inside the loop to
    stop gcc from recognizing a well-known pattern, which happens to work
    pretty fine. At least it allows us to make sure our local definition
    is not replaced with a self jump.
    
    The function only needs to be renamed back to strlen() so that the symbol
    exists, which implies that nolibc_strlen() which is used on variable
    strings has to be declared as a macro that points back to it before the
    strlen() macro is redifined.
    
    It was verified to produce valid code with gcc 3.4 to 12.1 at different
    optimization levels, and both with constant and variable strings.
    
    In case this problem surfaces again in the future, an alternate approach
    consisting in adding an optimize("no-tree-loop-distribute-patterns")
    function attribute for gcc>=12 worked as well but is less pretty.
    Reported-by: default avatarkernel test robot <yujie.liu@intel.com>
    Link: https://lore.kernel.org/r/202210081618.754a77db-yujie.liu@intel.com
    Fixes: 66b6f755 ("rcutorture: Import a copy of nolibc")
    Fixes: 96980b83 ("tools/nolibc/string: do not use __builtin_strlen() at -O0")
    Cc: "Paul E. McKenney" <paulmck@kernel.org>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Signed-off-by: default avatarWilly Tarreau <w@1wt.eu>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
    bfc3b0f0
string.h 5.03 KB