Hi, On Mon, 29 May 2017, Yuri Gribov wrote:
> Hi all, > > As discussed in > https://sourceware.org/ml/libc-alpha/2017-01/msg00455.html , some > libdl functions rely on return address to figure out the calling > DSO and then use this information in computation (e.g. output of dlsym > depends on which library called it). > > As reported in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66826 this > may break under tailcall optimization i.e. in cases like > > return dlsym(...); > > Carlos confirmed that they would prefer to have GCC attribute to > prevent tailcalls > (https://sourceware.org/ml/libc-alpha/2017-01/msg00502.html) so there > you go. A few comments: - the new attribute will need documentation - as mentioned earlier, calls to dlsym via a function pointer may still lead to the same issue (so the documentation should mention that) - this suppresses tailcalls for all dlsym calls, although only those with RTLD_NEXT are magic and need such suppression Are there any other possible uses for this attribute? Given the issue of calls-via-pointers, I don't understand why Glibc needs it, because for direct calls Jakub pointed out a simpler solution that works with existing compilers: #define dlsym(h, s) \ ({ \ void *__r = dlsym (h, s); \ asm ("" : "+r" (__r)); \ __r; }) (I think life would be easier for everyone if instead of making RTLD_NEXT magic, there was simply a way to look up a handle of the "next" dso...) Alexander
