Re: [PATCH] um: Abandon the _PAGE_NEWPROT bit

[Benjamin Berg]

Hi,

On Fri, 2024-10-11 at 13:38 +0800, Tiwei Bie wrote:
> When a PTE is updated in the page table, the _PAGE_NEWPAGE bit will
> always be set. And the corresponding page will always be mapped or
> unmapped depending on whether the PTE is present or not. The check
> on the _PAGE_NEWPROT bit is not really reachable. Abandoning it will
> allow us to simplify the code and remove the unreachable code.

Oh, nice cleanup!

And I like that mprotect is gone as I don't want it in SECCOMP mode :-)

Maybe we should rename _PAGE_NEWPAGE to something like _PAGE_NEEDSYNC?
I think that might make it more clear how everything ties together.

Anyway, the change looks good to me.

Benjamin

Reviewed-by: Benjamin Berg <benjamin.berg@xxxxxxxxx>

> Signed-off-by: Tiwei Bie <tiwei.btw@xxxxxxxxxxxx>
> ---
>  arch/um/include/asm/pgtable.h           | 40 ++++-----------
>  arch/um/include/shared/os.h             |  2 -
>  arch/um/include/shared/skas/stub-data.h |  1 -
>  arch/um/kernel/skas/stub.c              | 10 ----
>  arch/um/kernel/tlb.c                    | 66 +++++++++++------------
> --
>  arch/um/os-Linux/skas/mem.c             | 21 --------
>  6 files changed, 37 insertions(+), 103 deletions(-)
> 
> diff --git a/arch/um/include/asm/pgtable.h
> b/arch/um/include/asm/pgtable.h
> index bd7a9593705f..a32424cfe792 100644
> --- a/arch/um/include/asm/pgtable.h
> +++ b/arch/um/include/asm/pgtable.h
> @@ -12,7 +12,6 @@
>  
>  #define _PAGE_PRESENT	0x001
>  #define _PAGE_NEWPAGE	0x002
> -#define _PAGE_NEWPROT	0x004
>  #define _PAGE_RW	0x020
>  #define _PAGE_USER	0x040
>  #define _PAGE_ACCESSED	0x080
> @@ -151,23 +150,12 @@ static inline int pte_newpage(pte_t pte)
>  	return pte_get_bits(pte, _PAGE_NEWPAGE);
>  }
>  
> -static inline int pte_newprot(pte_t pte)
> -{
> -	return(pte_present(pte) && (pte_get_bits(pte,
> _PAGE_NEWPROT)));
> -}
> -
>  /*
>   * =================================
>   * Flags setting section.
>   * =================================
>   */
>  
> -static inline pte_t pte_mknewprot(pte_t pte)
> -{
> -	pte_set_bits(pte, _PAGE_NEWPROT);
> -	return(pte);
> -}
> -
>  static inline pte_t pte_mkclean(pte_t pte)
>  {
>  	pte_clear_bits(pte, _PAGE_DIRTY);
> @@ -184,17 +172,14 @@ static inline pte_t pte_wrprotect(pte_t pte)
>  {
>  	if (likely(pte_get_bits(pte, _PAGE_RW)))
>  		pte_clear_bits(pte, _PAGE_RW);
> -	else
> -		return pte;
> -	return(pte_mknewprot(pte));
> +	return pte;
>  }
>  
>  static inline pte_t pte_mkread(pte_t pte)
>  {
> -	if (unlikely(pte_get_bits(pte, _PAGE_USER)))
> -		return pte;
> -	pte_set_bits(pte, _PAGE_USER);
> -	return(pte_mknewprot(pte));
> +	if (likely(!pte_get_bits(pte, _PAGE_USER)))
> +		pte_set_bits(pte, _PAGE_USER);
> +	return pte;
>  }
>  
>  static inline pte_t pte_mkdirty(pte_t pte)
> @@ -211,18 +196,15 @@ static inline pte_t pte_mkyoung(pte_t pte)
>  
>  static inline pte_t pte_mkwrite_novma(pte_t pte)
>  {
> -	if (unlikely(pte_get_bits(pte,  _PAGE_RW)))
> -		return pte;
> -	pte_set_bits(pte, _PAGE_RW);
> -	return(pte_mknewprot(pte));
> +	if (likely(!pte_get_bits(pte, _PAGE_RW)))
> +		pte_set_bits(pte, _PAGE_RW);
> +	return pte;
>  }
>  
>  static inline pte_t pte_mkuptodate(pte_t pte)
>  {
>  	pte_clear_bits(pte, _PAGE_NEWPAGE);
> -	if(pte_present(pte))
> -		pte_clear_bits(pte, _PAGE_NEWPROT);
> -	return(pte);
> +	return pte;
>  }
>  
>  static inline pte_t pte_mknewpage(pte_t pte)
> @@ -236,12 +218,10 @@ static inline void set_pte(pte_t *pteptr, pte_t
> pteval)
>  	pte_copy(*pteptr, pteval);
>  
>  	/* If it's a swap entry, it needs to be marked _PAGE_NEWPAGE
> so
> -	 * fix_range knows to unmap it.  _PAGE_NEWPROT is specific
> to
> -	 * mapped pages.
> +	 * update_pte_range knows to unmap it.
>  	 */
>  
>  	*pteptr = pte_mknewpage(*pteptr);
> -	if(pte_present(*pteptr)) *pteptr = pte_mknewprot(*pteptr);
>  }
>  
>  #define PFN_PTE_SHIFT		PAGE_SHIFT
> @@ -298,8 +278,6 @@ static inline int pte_same(pte_t pte_a, pte_t
> pte_b)
>  	({ pte_t pte;					\
>  							\
>  	pte_set_val(pte, page_to_phys(page), (pgprot));	\
> -	if (pte_present(pte))				\
> -		pte_mknewprot(pte_mknewpage(pte));	\
>  	pte;})
>  
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> diff --git a/arch/um/include/shared/os.h
> b/arch/um/include/shared/os.h
> index bf539fee7831..09f8201de5db 100644
> --- a/arch/um/include/shared/os.h
> +++ b/arch/um/include/shared/os.h
> @@ -279,8 +279,6 @@ int map(struct mm_id *mm_idp, unsigned long virt,
>  	unsigned long len, int prot, int phys_fd,
>  	unsigned long long offset);
>  int unmap(struct mm_id *mm_idp, unsigned long addr, unsigned long
> len);
> -int protect(struct mm_id *mm_idp, unsigned long addr,
> -	    unsigned long len, unsigned int prot);
>  
>  /* skas/process.c */
>  extern int is_skas_winch(int pid, int fd, void *data);
> diff --git a/arch/um/include/shared/skas/stub-data.h
> b/arch/um/include/shared/skas/stub-data.h
> index 3fbdda727373..81a4cace032c 100644
> --- a/arch/um/include/shared/skas/stub-data.h
> +++ b/arch/um/include/shared/skas/stub-data.h
> @@ -30,7 +30,6 @@ enum stub_syscall_type {
>  	STUB_SYSCALL_UNSET = 0,
>  	STUB_SYSCALL_MMAP,
>  	STUB_SYSCALL_MUNMAP,
> -	STUB_SYSCALL_MPROTECT,
>  };
>  
>  struct stub_syscall {
> diff --git a/arch/um/kernel/skas/stub.c b/arch/um/kernel/skas/stub.c
> index 5d52ffa682dc..796fc266d3bb 100644
> --- a/arch/um/kernel/skas/stub.c
> +++ b/arch/um/kernel/skas/stub.c
> @@ -35,16 +35,6 @@ static __always_inline int syscall_handler(struct
> stub_data *d)
>  				return -1;
>  			}
>  			break;
> -		case STUB_SYSCALL_MPROTECT:
> -			res = stub_syscall3(__NR_mprotect,
> -					    sc->mem.addr, sc-
> >mem.length,
> -					    sc->mem.prot);
> -			if (res) {
> -				d->err = res;
> -				d->syscall_data_len = i;
> -				return -1;
> -			}
> -			break;
>  		default:
>  			d->err = -95; /* EOPNOTSUPP */
>  			d->syscall_data_len = i;
> diff --git a/arch/um/kernel/tlb.c b/arch/um/kernel/tlb.c
> index 548af31d4111..23c1f550cd7c 100644
> --- a/arch/um/kernel/tlb.c
> +++ b/arch/um/kernel/tlb.c
> @@ -23,9 +23,6 @@ struct vm_ops {
>  		    int phys_fd, unsigned long long offset);
>  	int (*unmap)(struct mm_id *mm_idp,
>  		     unsigned long virt, unsigned long len);
> -	int (*mprotect)(struct mm_id *mm_idp,
> -			unsigned long virt, unsigned long len,
> -			unsigned int prot);
>  };
>  
>  static int kern_map(struct mm_id *mm_idp,
> @@ -44,15 +41,6 @@ static int kern_unmap(struct mm_id *mm_idp,
>  	return os_unmap_memory((void *)virt, len);
>  }
>  
> -static int kern_mprotect(struct mm_id *mm_idp,
> -			 unsigned long virt, unsigned long len,
> -			 unsigned int prot)
> -{
> -	return os_protect_memory((void *)virt, len,
> -				 prot & UM_PROT_READ, prot &
> UM_PROT_WRITE,
> -				 1);
> -}
> -
>  void report_enomem(void)
>  {
>  	printk(KERN_ERR "UML ran out of memory on the host side! "
> @@ -65,33 +53,37 @@ static inline int update_pte_range(pmd_t *pmd,
> unsigned long addr,
>  				   struct vm_ops *ops)
>  {
>  	pte_t *pte;
> -	int r, w, x, prot, ret = 0;
> +	int ret = 0;
>  
>  	pte = pte_offset_kernel(pmd, addr);
>  	do {
> -		r = pte_read(*pte);
> -		w = pte_write(*pte);
> -		x = pte_exec(*pte);
> -		if (!pte_young(*pte)) {
> -			r = 0;
> -			w = 0;
> -		} else if (!pte_dirty(*pte))
> -			w = 0;
> -
> -		prot = ((r ? UM_PROT_READ : 0) | (w ? UM_PROT_WRITE
> : 0) |
> -			(x ? UM_PROT_EXEC : 0));
> -		if (pte_newpage(*pte)) {
> -			if (pte_present(*pte)) {
> -				__u64 offset;
> -				unsigned long phys = pte_val(*pte) &
> PAGE_MASK;
> -				int fd = phys_mapping(phys,
> &offset);
> -
> -				ret = ops->mmap(ops->mm_idp, addr,
> PAGE_SIZE,
> -						prot, fd, offset);
> -			} else
> -				ret = ops->unmap(ops->mm_idp, addr,
> PAGE_SIZE);
> -		} else if (pte_newprot(*pte))
> -			ret = ops->mprotect(ops->mm_idp, addr,
> PAGE_SIZE, prot);
> +		if (!pte_newpage(*pte))
> +			continue;
> +
> +		if (pte_present(*pte)) {
> +			__u64 offset;
> +			unsigned long phys = pte_val(*pte) &
> PAGE_MASK;
> +			int fd = phys_mapping(phys, &offset);
> +			int r, w, x, prot;
> +
> +			r = pte_read(*pte);
> +			w = pte_write(*pte);
> +			x = pte_exec(*pte);
> +			if (!pte_young(*pte)) {
> +				r = 0;
> +				w = 0;
> +			} else if (!pte_dirty(*pte))
> +				w = 0;
> +
> +			prot = (r ? UM_PROT_READ : 0) |
> +			       (w ? UM_PROT_WRITE : 0) |
> +			       (x ? UM_PROT_EXEC : 0);
> +
> +			ret = ops->mmap(ops->mm_idp, addr,
> PAGE_SIZE,
> +					prot, fd, offset);
> +		} else
> +			ret = ops->unmap(ops->mm_idp, addr,
> PAGE_SIZE);
> +
>  		*pte = pte_mkuptodate(*pte);
>  	} while (pte++, addr += PAGE_SIZE, ((addr < end) && !ret));
>  	return ret;
> @@ -180,11 +172,9 @@ int um_tlb_sync(struct mm_struct *mm)
>  	if (mm == &init_mm) {
>  		ops.mmap = kern_map;
>  		ops.unmap = kern_unmap;
> -		ops.mprotect = kern_mprotect;
>  	} else {
>  		ops.mmap = map;
>  		ops.unmap = unmap;
> -		ops.mprotect = protect;
>  	}
>  
>  	pgd = pgd_offset(mm, addr);
> diff --git a/arch/um/os-Linux/skas/mem.c b/arch/um/os-
> Linux/skas/mem.c
> index 9a13ac23c606..d7f1814b0e5a 100644
> --- a/arch/um/os-Linux/skas/mem.c
> +++ b/arch/um/os-Linux/skas/mem.c
> @@ -217,24 +217,3 @@ int unmap(struct mm_id *mm_idp, unsigned long
> addr, unsigned long len)
>  
>  	return 0;
>  }
> -
> -int protect(struct mm_id *mm_idp, unsigned long addr, unsigned long
> len,
> -	    unsigned int prot)
> -{
> -	struct stub_syscall *sc;
> -
> -	/* Compress with previous syscall if that is possible */
> -	sc = syscall_stub_get_previous(mm_idp,
> STUB_SYSCALL_MPROTECT, addr);
> -	if (sc && sc->mem.prot == prot) {
> -		sc->mem.length += len;
> -		return 0;
> -	}
> -
> -	sc = syscall_stub_alloc(mm_idp);
> -	sc->syscall = STUB_SYSCALL_MPROTECT;
> -	sc->mem.addr = addr;
> -	sc->mem.length = len;
> -	sc->mem.prot = prot;
> -
> -	return 0;
> -}