Re: [PATCH v2] fs/proc/kcore.c: add mmap interface

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  1 Jun 2021 16:22:41 +0800 Feng zhou <zhoufeng.zf@bytedance.com> wrote:

> From: ZHOUFENG <zhoufeng.zf@bytedance.com>
> 
> When we do the kernel monitor, use the DRGN
> (https://github.com/osandov/drgn) access to kernel data structures,
> found that the system calls a lot. DRGN is implemented by reading
> /proc/kcore. After looking at the kcore code, it is found that kcore
> does not implement mmap, resulting in frequent context switching
> triggered by read. Therefore, we want to add mmap interface to optimize
> performance. Since vmalloc and module areas will change with allocation
> and release, consistency cannot be guaranteed, so mmap interface only
> maps KCORE_TEXT and KCORE_RAM.
> 
> ...
>
> +static int mmap_kcore(struct file *file, struct vm_area_struct *vma)
> +{
> +	size_t size = vma->vm_end - vma->vm_start;
> +	u64 start, pfn;
> +	int nphdr;
> +	size_t data_offset;
> +	size_t phdrs_len, notes_len;
> +	struct kcore_list *m = NULL;
> +	int ret = 0;
> +
> +	down_read(&kclist_lock);
> +
> +	get_kcore_size(&nphdr, &phdrs_len, &notes_len, &data_offset);
> +
> +	start = kc_offset_to_vaddr(((u64)vma->vm_pgoff << PAGE_SHIFT) -
> +		((data_offset >> PAGE_SHIFT) << PAGE_SHIFT));
> +
> +	list_for_each_entry(m, &kclist_head, list) {
> +		if (start >= m->addr && size <= m->size)
> +			break;
> +	}
> +
> +	if (&m->list == &kclist_head) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	if (vma->vm_flags & (VM_WRITE | VM_EXEC)) {
> +		ret = -EPERM;
> +		goto out;
> +	}
> +
> +	vma->vm_flags &= ~(VM_MAYWRITE | VM_MAYEXEC);
> +	vma->vm_flags |= VM_MIXEDMAP;
> +	vma->vm_ops = &kcore_mmap_ops;
> +
> +	if (kern_addr_valid(start)) {
> +		if (m->type == KCORE_RAM || m->type == KCORE_REMAP)

KCORE_REMAP was removed by
https://lkml.kernel.org/r/20210526093041.8800-2-david@redhat.com

I did this:

--- a/fs/proc/kcore.c~fs-proc-kcorec-add-mmap-interface-fix
+++ a/fs/proc/kcore.c
@@ -660,7 +660,7 @@ static int mmap_kcore(struct file *file,
 	vma->vm_ops = &kcore_mmap_ops;
 
 	if (kern_addr_valid(start)) {
-		if (m->type == KCORE_RAM || m->type == KCORE_REMAP)
+		if (m->type == KCORE_RAM)
 			pfn = __pa(start) >> PAGE_SHIFT;
 		else if (m->type == KCORE_TEXT)
 			pfn = __pa_symbol(start) >> PAGE_SHIFT;