Re: [PATCH v6 13/25] powerpc: Remove direct call to mmap2 syscall handlers

On Fri, Sep 30, 2022, at 3:19 PM, Michael Ellerman wrote:

"Arnd Bergmann" [off-list ref] writes:

quoted

While sys_mmap_pgoff() was meant to replace the various sys_mmap2()
implementations, I think it was ultimately a mistake, and we later
converged on the sys_mmap2() calling conventions with 12 bits
offset for almost all 32-bit architectures.

I only see 3 compat mmap2s:

  $ gg "COMPAT_SYSCALL.*mmap2"
  arch/arm64/kernel/sys32.c:COMPAT_SYSCALL_DEFINE6(aarch32_mmap2, 
unsigned long, addr, unsigned long, len,
  arch/powerpc/kernel/syscalls.c:COMPAT_SYSCALL_DEFINE6(mmap2, unsigned 
long, addr, size_t, len,
  arch/s390/kernel/compat_linux.c:COMPAT_SYSCALL_DEFINE1(s390_mmap2, 
struct mmap_arg_struct_emu31 __user *, arg)

They are all inconsistently named, and some are shared with the 64-bit
implementation on architectures that provide mmap2 for both 32-bit
and 64-bit mode, rather than only for 32-bit.

arch/mips/kernel/syscall.c:SYSCALL_DEFINE6(mips_mmap2, unsigned long, addr, unsigned long, len,
arch/sparc/kernel/sys32.S:      .globl          sys32_mmap2
arch/parisc/kernel/sys_parisc.c:asmlinkage unsigned long sys_mmap2(unsigned long addr, unsigned long len,

s390 is weird.

Right, they used to be limited to 5 register arguments

The arm64 one and ours are similar, but we have the additional call to
arch_validate_prot(prot, addr). arm64 does implement arch_validate_prot().

Similar with mmap2, we call arch_validate_prot() but no one else does 
(why not?).

This looks like it was added in ef3d3246a0d0 ("powerpc/mm: Add Strong
Access Ordering support"), which is powerpc specific. It looks like
this should correspond to a custom arch_calc_vm_prot_bits()
implementation, which exists on arm64, powerpc, sparc and x86.

I suppose it should be there for those four.

    Arnd