On Wed, 3 Oct 2007 22:20:46 +0900, KAMEZAWA Hiroyuki wrote: > On Wed, 3 Oct 2007 21:40:29 +0900 > KAMEZAWA Hiroyuki <[EMAIL PROTECTED]> wrote: > > > On Wed, 3 Oct 2007 14:20:07 +0200 (MEST) > > Mikael Pettersson <[EMAIL PROTECTED]> wrote: > > > > > What I don't agree with is the logic itself: > > > - You only catch altstack overflow caused by the kernel pushing > > > a sigframe. You don't catch overflow caused by the user-space > > > signal handler pushing its own stack frame after the sigframe. > > > - SUSv3 specifies the effect of altstack overflow as "undefined". > > > - The overflow problem can be solved in user-space: allocate the > > > altstack with mmap(), then mprotect() the lowest page to prevent > > > accesses to it. Any overflow into it, by the kernel's signal > > > delivery code or by the user-space signal handler, will be caught. > > > > > > So this patch gets a NAK from me. > > > > > > > I can understand what you say, but a program which meets this problem > > cannot be debugged ;( > > > > gdb just shows infinit loop of function frames and origignal signal frame > > which includes the most important information is overwritten. > > > there is a difference among user's stack overflow and kernel's. > - user's stack overflow just breaks memory next to stack frame. > - kernel's altstack overflow, which this patch tries to fix, breaks > the bottom of altstack bacause %esp goes back to the bottom > of ths altstack when it exceeds altstack range. > This behavior overwrite orignail stack frame and shows infinit loop > of function call to gdb and never stop with 100% cpu usage.
The proposed kernel signal delivery patch only handles the case where the /sigframe/ ends up overlapping the end of the altstack. If the sigframe remains within the altstack boundaries but the user-space signal handler adds an /ordinary stack frame/ that moves SP beyond the altstack limit, then the kernel patch solves nothing and recursive signals will cause altstack wraparound. On the other hand, the user-space technique of making the lowest page(s) in the altstack inaccessible handles both cases of overflow. /Mikael - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
