On Wed, Jan 16, 2002 at 08:00:17AM +0000, Mark Zealey wrote: > On Wed, Jan 16, 2002 at 05:44:56AM +0000, John Levon wrote: > > > > > can someone explain the purpose of this code : > > > > #define RESTORE_ALL \ > > [snip] > > 1: popl %ds; \ > > 2: popl %es; \ > > addl $4,%esp; \ > > 3: iret; \ > > .section .fixup,"ax"; \ > > 4: movl $0,(%esp); \ > > jmp 1b; \ > > 5: movl $0,(%esp); \ > > jmp 2b; \ > > 6: pushl %ss; \ > > popl %ds; \ > > pushl %ss; \ > > popl %es; \ > > pushl $11; \ > > call do_exit; \ > > .previous; \ > > .section __ex_table,"a";\ > > .align 4; \ > > .long 1b,4b; \ > > .long 2b,5b; \ > > .long 3b,6b; \ > > > > Now, I understand the exception table stuff (fault at 1 ends up at 4 etc.) but I am > > unable to fathom what it's actually for. Ideas ? > > OK, I was gonna say 'I dunno' but, looking closer at it, it looks like it > handles the cases where we were called from an invalid ds or es, or if we can't > iret (bad cs/esp/ss/eip/whatever). In the first 2, it just writes 0 to them, in > the last case, it kills the process. Hmm, it's sort of described in "Understanding the linux kernel" (page 134), but nothing about the .fixup ane __ex_table sections. However, from a long, long time ago in a galaxy far, far away, I remember that at the start of a new development series (I think it was linux-2.1) Linus modified the exception handlers in such a way that they could handle exceptions from kernel mode. In the old days you had to call verify_area() before you touched a userland buffer with copy_from_user() et. al. That was slow because the kernel actually checked the process memory space to see if the memory was valid, and in 99% of the cases the memory *was* indeed safe to access. That made it a prime candidate for speeding up, and the __ex_table section magic exactly did that. I think you can still find the magic explained in a linux-kernel archive, though I think it we used news:comp.os.linux.development or news:linux.dev.kernel at that time. I remember posting a message with benchmark results at that time, so you could try to look for my name. I think it was somewhere around linux-2.1.4, but it could also be around linux-1.3.4. Erik -- J.A.K. (Erik) Mouw, Information and Communication Theory Group, Faculty of Information Technology and Systems, Delft University of Technology, PO BOX 5031, 2600 GA Delft, The Netherlands Phone: +31-15-2783635 Fax: +31-15-2781843 Email: J.A.K.Mouw@its.tudelft.nl WWW: http://www-ict.its.tudelft.nl/~erik/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
