On Mon, Feb 10, 2025 at 2:23 PM Jordan Rome <linux@xxxxxxxxxxxxxx> wrote:
>
> Similar to `access_process_vm` but specific to strings.
> Also chunks reads by page and utilizes `strscpy`
> for handling null termination.
>
> Signed-off-by: Jordan Rome <linux@xxxxxxxxxxxxxx>
> ---
> include/linux/mm.h | 3 ++
> mm/memory.c | 119 +++++++++++++++++++++++++++++++++++++++++++++
> mm/nommu.c | 73 +++++++++++++++++++++++++++
> 3 files changed, 195 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7b1068ddcbb7..aee23d84ce01 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2486,6 +2486,9 @@ extern int access_process_vm(struct task_struct *tsk, unsigned long addr,
> extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
> void *buf, int len, unsigned int gup_flags);
>
> +extern int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr,
> + void *buf, int len, unsigned int gup_flags);
> +
> long get_user_pages_remote(struct mm_struct *mm,
> unsigned long start, unsigned long nr_pages,
> unsigned int gup_flags, struct page **pages,
> diff --git a/mm/memory.c b/mm/memory.c
> index 539c0f7c6d54..e9d8584a7f56 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6803,6 +6803,125 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr,
> }
> EXPORT_SYMBOL_GPL(access_process_vm);
>
> +/*
> + * Copy a string from another process's address space as given in mm.
> + * If there is any error return -EFAULT.
> + */
> +static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
> + void *buf, int len, unsigned int gup_flags)
> +{
> + void *old_buf = buf;
> + int err = 0;
> +
> + *(char *)buf = '\0';
LGTM overall:
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
But note that all this unconditional buf access will be incorrect if
len == 0. So either all of that has to be guarded with `if (len)`,
just dropped, or declared unsupported, depending on what mm folks
think. BPF helper won't ever call with len == 0, so that's why my ack.
(And yes, it would be nice to hear from someone from the MM side at
this point, thank you!).
> +
> + if (mmap_read_lock_killable(mm))
> + return -EFAULT;
> +
> + /* Untag the address before looking up the VMA */
> + addr = untagged_addr_remote(mm, addr);
> +
> + /* Avoid triggering the temporary warning in __get_user_pages */
> + if (!vma_lookup(mm, addr)) {
> + err = -EFAULT;
> + goto out;
> + }
> +
[...]
