On Mon, Jan 6, 2025 at 6:12 PM Jordan Rome <linux@xxxxxxxxxxxxxx> wrote:
>
> This new kfunc will be able to copy a string
> from another process's/task's address space.
nit: this is kernel code, task is unambiguous, so I'd drop the
"process" reference here
> This is similar to `bpf_copy_from_user_str`
> but accepts a `struct task_struct*` argument.
>
> This required adding an additional function
> in memory.c, namely `copy_str_from_process_vm`,
> which works similar to `access_process_vm`
> but utilizes the `strncpy_from_user` helper
> and only supports reading/copying and not writing.
>
> Signed-off-by: Jordan Rome <linux@xxxxxxxxxxxxxx>
> ---
> include/linux/mm.h | 3 ++
> kernel/bpf/helpers.c | 46 ++++++++++++++++++++
> mm/memory.c | 101 +++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 150 insertions(+)
>
please check kernel test bot's complains as well
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c39c4945946c..52b304b20630 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2484,6 +2484,9 @@ 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_str_from_process_vm(struct task_struct *tsk, unsigned long addr,
> + void *buf, int len, unsigned int gup_flags);
nit: curious what mm folks think about naming, I'd go with a slightly
less verbose naming: "copy_remote_vm_str" (copy vs access, _str suffix
for non fixed-sized semantics marking)
for the next revision, let's split out mm parts from helpers parts, I
don't think we lose much as this new internal API is self-contained,
and it will be easier for mm folks to review
> +
> 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/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index cd5f9884d85b..45d41b7a9906 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3072,6 +3072,51 @@ __bpf_kfunc void bpf_local_irq_restore(unsigned long *flags__irq_flag)
> local_irq_restore(*flags__irq_flag);
> }
>
> +/**
> + * bpf_copy_from_user_task_str() - Copy a string from an task's address space
> + * @dst: Destination address, in kernel space. This buffer must be
> + * at least @dst__sz bytes long.
> + * @dst__sz: Maximum number of bytes to copy, includes the trailing NUL.
> + * @unsafe_ptr__ign: Source address in the task's address space.
> + * @tsk: The task whose address space will be used
> + * @flags: The only supported flag is BPF_F_PAD_ZEROS
> + *
> + * Copies a NULL-terminated string from a task's address space to BPF space.
there is no "BPF space", really... maybe "copies string into *dst* buffer"
> + * If user string is too long this will still ensure zero termination in the
> + * dst buffer unless buffer size is 0.
> + *
> + * If BPF_F_PAD_ZEROS flag is set, memset the tail of @dst to 0 on success and
> + * memset all of @dst on failure.
> + */
> +__bpf_kfunc int bpf_copy_from_user_task_str(void *dst, u32 dst__sz, const void __user *unsafe_ptr__ign, struct task_struct *tsk, u64 flags)
this looks like a long line, does it fit under 100 characters?
> +{
> + int count = dst__sz - 1;
> + int ret = 0;
> +
> + if (unlikely(flags & ~BPF_F_PAD_ZEROS))
> + return -EINVAL;
> +
> + if (unlikely(!dst__sz))
> + return 0;
> +
> + ret = copy_str_from_process_vm(tsk, (unsigned long)unsafe_ptr__ign, dst, count, 0);
> +
> + if (ret <= 0) {
> + if (flags & BPF_F_PAD_ZEROS)
> + memset((char *)dst, 0, dst__sz);
nit: no need for (char *) cast? memset takes void *, I think
> + return ret;
if ret == 0, is that an error? If so, `return ret ?: -EINVAL;` or
something along those lines?
pw-bot: cr
> + }
> +
> + if (ret < count) {
> + if (flags & BPF_F_PAD_ZEROS)
> + memset((char *)dst + ret, 0, dst__sz - ret);
> + } else {
> + ((char *)dst)[count] = '\0';
> + }
> +
> + return ret + 1;
> +}
> +
> __bpf_kfunc_end_defs();
>
> BTF_KFUNCS_START(generic_btf_ids)
> @@ -3164,6 +3209,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
> BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
> BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
> +BTF_ID_FLAGS(func, bpf_copy_from_user_task_str, KF_SLEEPABLE)
> BTF_ID_FLAGS(func, bpf_get_kmem_cache)
> BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE)
> BTF_ID_FLAGS(func, bpf_iter_kmem_cache_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)
> diff --git a/mm/memory.c b/mm/memory.c
> index 75c2dfd04f72..514490bd7d6d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6673,6 +6673,75 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
> return buf - old_buf;
> }
>
> +/*
> + * Copy a string from another process's address space as given in mm.
> + * Don't return partial results. If there is any error return -EFAULT.
What does "don't return partial results" mean? What happens if we read
part of a string and then fail to read the rest?
> + */
> +static int __copy_str_from_remote_vm(struct mm_struct *mm, unsigned long addr,
> + void *buf, int len, unsigned int gup_flags)
> +{
> + void *old_buf = buf;
> + int err = 0;
> +
> + 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)) {
> + mmap_read_unlock(mm);
> + return -EFAULT;
maybe let's do (so that we do mmap_read_unlock in just one place)
err = -EFAULT;
goto err_out;
and then see below
> + }
> +
> + while (len) {
> + int bytes, offset, retval;
> + void *maddr;
> + struct vm_area_struct *vma = NULL;
> + struct page *page = get_user_page_vma_remote(mm, addr,
> + gup_flags, &vma);
> +
nit: I'd split page declaration and assignment and kept
get_user_page_vma_remote() invocation on a single line
> + if (IS_ERR(page)) {
> + /*
> + * Treat as a total failure for now until we decide how
> + * to handle the CONFIG_HAVE_IOREMAP_PROT case and
> + * stack expansion.
> + */
> + err = -EFAULT;
> + break;
> + }
> +
> + bytes = len;
> + offset = addr & (PAGE_SIZE - 1);
> + if (bytes > PAGE_SIZE - offset)
> + bytes = PAGE_SIZE - offset;
> +
> + maddr = kmap_local_page(page);
> + retval = strncpy_from_user(buf, (const char __user *)addr, bytes);
you are not using maddr... that seems wrong (even if it works due to
how kmap_local_page is currently implemented)
> + unmap_and_put_page(page, maddr);
> +
> + if (retval < 0) {
> + err = retval;
> + break;
> + }
> +
> + len -= retval;
> + buf += retval;
> + addr += retval;
> +
> + /* Found the end of the string */
> + if (retval < bytes)
> + break;
> + }
err_out: here
> + mmap_read_unlock(mm);
> +
> + if (err)
> + return err;
> +
> + return buf - old_buf;
> +}
> +
> /**
> * access_remote_vm - access another process' address space
> * @mm: the mm_struct of the target address space
> @@ -6714,6 +6783,38 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr,
> }
> EXPORT_SYMBOL_GPL(access_process_vm);
>
> +/**
> + * copy_str_from_process_vm - copy a string from another process's address space.
> + * @tsk: the task of the target address space
> + * @addr: start address to access
access -> read from
> + * @buf: source or destination buffer
for this api it's always the destination, right?
> + * @len: number of bytes to transfer
> + * @gup_flags: flags modifying lookup behaviour
> + *
> + * The caller must hold a reference on @mm.
> + *
> + * Return: number of bytes copied from source to destination. If the string
> + * is shorter than @len then return the length of the string.
and if the string is longer than @len, then what happens? we should
either specify or drop the "if string is shorter bit" and make it
unambiguous whether terminating zero is included or not
> + * On any error, return -EFAULT.
> + */
> +int copy_str_from_process_vm(struct task_struct *tsk, unsigned long addr,
> + void *buf, int len, unsigned int gup_flags)
> +{
> + struct mm_struct *mm;
> + int ret;
> +
> + mm = get_task_mm(tsk);
> + if (!mm)
> + return -EFAULT;
> +
> + ret = __copy_str_from_remote_vm(mm, addr, buf, len, gup_flags);
> +
> + mmput(mm);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(copy_str_from_process_vm);
> +
> /*
> * Print the name of a VMA.
> */
> --
> 2.43.5
>
