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MIPS: Export syscall stack arguments properly for remote use

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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@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
This commit is contained in:
Maciej W. Rozycki 2025-02-11 18:22:30 +00:00 • committed by Thomas Bogendoerfer
parent 2014c95afe
commit ed975485a1
3 changed files with 12 additions and 6 deletions
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4
arch/mips/include/asm/ptrace.h
View file

@ -27,8 +27,8 @@
*/
struct pt_regs {
#ifdef CONFIG_32BIT
/* Pad bytes for argument save space on the stack. */
unsigned long pad0[8];
/* Saved syscall stack arguments; entries 0-3 unused. */
unsigned long args[8];
#endif
/* Saved main processor registers. */

6
arch/mips/kernel/asm-offsets.c
View file

@ -27,6 +27,12 @@ void output_ptreg_defines(void);
void output_ptreg_defines(void)
{
COMMENT("MIPS pt_regs offsets.");
#ifdef CONFIG_32BIT
OFFSET(PT_ARG4, pt_regs, args[4]);
OFFSET(PT_ARG5, pt_regs, args[5]);
OFFSET(PT_ARG6, pt_regs, args[6]);
OFFSET(PT_ARG7, pt_regs, args[7]);
#endif
OFFSET(PT_R0, pt_regs, regs[0]);
OFFSET(PT_R1, pt_regs, regs[1]);
OFFSET(PT_R2, pt_regs, regs[2]);

8
arch/mips/kernel/scall32-o32.S
View file

@ -64,10 +64,10 @@ load_a6: user_lw(t7, 24(t0)) # argument #7 from usp
load_a7: user_lw(t8, 28(t0)) # argument #8 from usp
loads_done:
sw t5, 16(sp) # argument #5 to ksp
sw t6, 20(sp) # argument #6 to ksp
sw t7, 24(sp) # argument #7 to ksp
sw t8, 28(sp) # argument #8 to ksp
sw t5, PT_ARG4(sp) # argument #5 to ksp
sw t6, PT_ARG5(sp) # argument #6 to ksp
sw t7, PT_ARG6(sp) # argument #7 to ksp
sw t8, PT_ARG7(sp) # argument #8 to ksp
.set pop
.section __ex_table,"a"