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Date: Sun, 30 Jan 2022 12:34:41 -0800 From: Kees Cook <keescook@...omium.org> To: Andy Shevchenko <andy.shevchenko@...il.com> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Peter Rosin <peda@...ntia.se>, Andy Shevchenko <andy@...nel.org>, Matteo Croce <mcroce@...rosoft.com>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev, linux-hardening@...r.kernel.org Subject: Re: [PATCH] lib/test_string.c: Add test for strlen() On Sun, Jan 30, 2022 at 08:56:40PM +0200, Andy Shevchenko wrote: > On Sun, Jan 30, 2022 at 8:36 PM Kees Cook <keescook@...omium.org> wrote: > > > > Add a simple test for strlen() functionality, including using it as a > > constant expression. > > ... > > > +/* > > + * Unlike many other string functions, strlen() can be used in > > + * static initializers when string lengths are known at compile > > + * time. (i.e. Under these conditions, strlen() is a constant > > + * expression.) Make sure it can be used this way. > > + */ > > +static const int strlen_ce = strlen("tada, a constant expression"); > > So, the compiler will replace this by a constant and then eliminate > the condition completely from the code. Did I understand this > correctly? Yup! See: https://godbolt.org/z/nTqPaszTh There a few rare places in the kernel that do this, which is how I noticed. (I broke strlen() with the recent FORTIFY changes.) > > +static __init int strlen_selftest(void) > > +{ > > + /* String length ruler: 123456789012345 */ > > + static const char normal[] = "I am normal"; > > + static const char *ptr = "where do I go?"; > > + static const char trailing[] = "hidden NULLs\0\0\0"; > > + static const char leading[] = "\0\0hidden text"; > > + > > + if (strlen(normal) != 11) > > + return 0x100001; > > + if (strlen(ptr++) != 14) > > + return 0x100002; > > + if (strlen(ptr++) != 13) > > + return 0x100003; > > + if (strlen(trailing) != 12) > > + return 0x100004; > > + if (strlen(leading) != 0) > > + return 0x100005; > > > + if (strlen_ce != 27) > > + return 0x100006; > > ...so this part won't ever appear in the assembly (assuming -O2). Correct, unless strlen() breaks. > Same to the rest? If so, why is this not a part of the compiler tests? I wanted to keep everything together -- this includes a macro side-effect test as well ("ptr++"). -Kees -- Kees Cook
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