On Tue, Feb 11, 2025 at 06:22:30PM +0000, Maciej W. Rozycki wrote: > We have several places across the kernel where we want to access another > task's syscall arguments, such as ptrace(2), seccomp(2), etc., by making > a call to syscall_get_arguments(). > > This works for register arguments right away by accessing the task's > `regs' member of `struct pt_regs', however for stack arguments seen with > 32-bit/o32 kernels things are more complicated. Technically they ought > to be obtained from the user stack with calls to an access_remote_vm(), > but we have an easier way available already. > > So as to be able to access syscall stack arguments as regular function > arguments following the MIPS calling convention we copy them over from > the user stack to the kernel stack in arch/mips/kernel/scall32-o32.S, in > handle_sys(), to the current stack frame's outgoing argument space at > the top of the stack, which is where the handler called expects to see > its incoming arguments. This area is also pointed at by the `pt_regs' > pointer obtained by task_pt_regs(). > > Make the o32 stack argument space a proper member of `struct pt_regs' > then, by renaming the existing member from `pad0' to `args' and using > generated offsets to access the space. No functional change though. > > With the change in place the o32 kernel stack frame layout at the entry > to a syscall handler invoked by handle_sys() is therefore as follows: > > $sp + 68 -> | ... | <- pt_regs.regs[9] > +---------------------+ > $sp + 64 -> | $t0 | <- pt_regs.regs[8] > +---------------------+ > $sp + 60 -> | $a3/argument #4 | <- pt_regs.regs[7] > +---------------------+ > $sp + 56 -> | $a2/argument #3 | <- pt_regs.regs[6] > +---------------------+ > $sp + 52 -> | $a1/argument #2 | <- pt_regs.regs[5] > +---------------------+ > $sp + 48 -> | $a0/argument #1 | <- pt_regs.regs[4] > +---------------------+ > $sp + 44 -> | $v1 | <- pt_regs.regs[3] > +---------------------+ > $sp + 40 -> | $v0 | <- pt_regs.regs[2] > +---------------------+ > $sp + 36 -> | $at | <- pt_regs.regs[1] > +---------------------+ > $sp + 32 -> | $zero | <- pt_regs.regs[0] > +---------------------+ > $sp + 28 -> | stack argument #8 | <- pt_regs.args[7] > +---------------------+ > $sp + 24 -> | stack argument #7 | <- pt_regs.args[6] > +---------------------+ > $sp + 20 -> | stack argument #6 | <- pt_regs.args[5] > +---------------------+ > $sp + 16 -> | stack argument #5 | <- pt_regs.args[4] > +---------------------+ > $sp + 12 -> | psABI space for $a3 | <- pt_regs.args[3] > +---------------------+ > $sp + 8 -> | psABI space for $a2 | <- pt_regs.args[2] > +---------------------+ > $sp + 4 -> | psABI space for $a1 | <- pt_regs.args[1] > +---------------------+ > $sp + 0 -> | psABI space for $a0 | <- pt_regs.args[0] > +---------------------+ > > holding user data received and with the first 4 frame slots reserved by > the psABI for the compiler to spill the incoming arguments from $a0-$a3 > registers (which it sometimes does according to its needs) and the next > 4 frame slots designated by the psABI for any stack function arguments > that follow. This data is also available for other tasks to peek/poke > at as reqired and where permitted. > > Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx> > --- > arch/mips/include/asm/ptrace.h | 4 ++-- > arch/mips/kernel/asm-offsets.c | 6 ++++++ > arch/mips/kernel/scall32-o32.S | 8 ++++---- > 3 files changed, 12 insertions(+), 6 deletions(-) applied to mips-fixes Thomas. -- Crap can work. Given enough thrust pigs will fly, but it's not necessarily a good idea. [ RFC1925, 2.3 ]
