On 2018-10-30 21:58, Kees Cook wrote: > On Sat, Oct 27, 2018 at 12:24 AM, Rasmus Villemoes > <linux@xxxxxxxxxxxxxxxxxx> wrote: > > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=ce9b938574042d09920650cb3c63ec29658edc87 > The above seemed to "noisy" to send, but perhaps we should just land > it anyway. They really _should_ be const. > Isn't that 063246641d4a9e9de84a2466fbad50112faf88dc in mainline ;) ? BTW, I don't agree with all the changes in there: For auto variables, this - const char *cur_drv, *drv = "acpi-cpufreq"; + const char drv[] = "acpi-cpufreq"; + const char *cur_drv; makes gcc actually generate that string on the stack instead of just referring to an anonymous object in .rodata; one gets code gen like +: 31 c0 xor %eax,%eax +: 48 b8 61 63 70 69 2d movabs $0x7570632d69706361,%rax # "acpi-cpu" +: 63 70 75 +: c7 44 24 0b 66 72 65 movl $0x71657266,0xb(%rsp) # "freq" +: 71 +: c6 44 24 0f 00 movb $0x0,0xf(%rsp) "\0" +: 48 89 44 24 03 mov %rax,0x3(%rsp) It's not the-end-of-the-world-horrible, but it's better avoided, especially for patches that are not supposed to change anything. And longer strings would of course produce even more gunk like the above. A better fix which also silences -Wformat-security is to declare the variable itself const, i.e. const char *const drv = "acpi-cpufreq". Yes, gcc should be able to infer the constness of drv from the fact that it's never assigned to elsewhere in the function... I think I saw that on some gcc todo list at some point. > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=b7dcfc8f48caaafcc423e5793f7ef61b9bb5c458 > This one covers cases where the pointer is pointing to a const string, > so really there's no sense in injecting the "%s", but I was collecting > them to make real ones stand out. I don't agree. Yes, a human can verify that _currently_, only "pencrypt" and "pdecrypt" can ever reach pcrypt_sysfs_add(). But without the compiler being smart enough to do that, one will never know if some new caller shows up, or one of those literals grows a % for some reason. Adding "%s" doesn't cost much, especially not in cases (like this one) where the fmt+args end up at kobject_set_name_vargs - for a "%s" + literal that does a (succesful) kstrdup_const(), so we never even hit the vsnprintf() engine. >> Patches 5,6,7 are >> some examples of where one might add fmtcheck() calls. I don't think >> we can get to a state where we can unconditionally add >> -Wformat-nonliteral to the build, but I think there's a lot of >> low-hanging fruit. > > How much work do you think it'd take to get to a > format-nonliteral-clean build? I think it's worth doing the work if > it's not totally intractable. Probably less than the VLA removal. But it kind of depends on which tools one allows. I can't see how to do it without something like fmtcheck() to annotate certain places (e.g. the nfs example). Maybe a no_fmtcheck() to annotate places which have been manually verified [modulo the above "but that may change..."] would also be needed (no_fmtcheck would be the same as fmtcheck for at !CONFIG_FMTCHECK kernel), similar to how we have no_printk. I kind of agree with Guenther that the hwmon example is a bad one. It would be better to have the compiler check all those string literals against a pattern at build time. Probably the format template plugin can be extended to apply to any "const char*" declaration, not just those sitting inside structs. But I'd rather get fmtcheck() in first before returning to work on that plugin. Rasmus
