Re: [PATCH v4 2/4] fs/proc/kcore: convert read_kcore() to read_kcore_iter()

[Baoquan He]

Hi Lorenzo,

On 03/21/23 at 08:54pm, Lorenzo Stoakes wrote:
> Now we have eliminated spinlocks from the vread() case, convert
> read_kcore() to read_kcore_iter().

Sorry for late comment.

Here I could miss something important, I don't get where we have
eliminated spinlocks from the vread() case. Do I misunderstand this
sentence?

> 
> For the time being we still use a bounce buffer for vread(), however in the
> next patch we will convert this to interact directly with the iterator and
> eliminate the bounce buffer altogether.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
>  fs/proc/kcore.c | 58 ++++++++++++++++++++++++-------------------------
>  1 file changed, 29 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 556f310d6aa4..25e0eeb8d498 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -24,7 +24,7 @@
>  #include <linux/memblock.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
> -#include <linux/uaccess.h>
> +#include <linux/uio.h>
>  #include <asm/io.h>
>  #include <linux/list.h>
>  #include <linux/ioport.h>
> @@ -308,9 +308,12 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
>  }
>  
>  static ssize_t
> -read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
> +read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>  {
> +	struct file *file = iocb->ki_filp;
>  	char *buf = file->private_data;
> +	loff_t *ppos = &iocb->ki_pos;
> +
>  	size_t phdrs_offset, notes_offset, data_offset;
>  	size_t page_offline_frozen = 1;
>  	size_t phdrs_len, notes_len;
> @@ -318,6 +321,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  	size_t tsz;
>  	int nphdr;
>  	unsigned long start;
> +	size_t buflen = iov_iter_count(iter);
>  	size_t orig_buflen = buflen;
>  	int ret = 0;
>  
> @@ -333,7 +337,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  	notes_offset = phdrs_offset + phdrs_len;
>  
>  	/* ELF file header. */
> -	if (buflen && *fpos < sizeof(struct elfhdr)) {
> +	if (buflen && *ppos < sizeof(struct elfhdr)) {
>  		struct elfhdr ehdr = {
>  			.e_ident = {
>  				[EI_MAG0] = ELFMAG0,
> @@ -355,19 +359,18 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  			.e_phnum = nphdr,
>  		};
>  
> -		tsz = min_t(size_t, buflen, sizeof(struct elfhdr) - *fpos);
> -		if (copy_to_user(buffer, (char *)&ehdr + *fpos, tsz)) {
> +		tsz = min_t(size_t, buflen, sizeof(struct elfhdr) - *ppos);
> +		if (copy_to_iter((char *)&ehdr + *ppos, tsz, iter) != tsz) {
>  			ret = -EFAULT;
>  			goto out;
>  		}
>  
> -		buffer += tsz;
>  		buflen -= tsz;
> -		*fpos += tsz;
> +		*ppos += tsz;
>  	}
>  
>  	/* ELF program headers. */
> -	if (buflen && *fpos < phdrs_offset + phdrs_len) {
> +	if (buflen && *ppos < phdrs_offset + phdrs_len) {
>  		struct elf_phdr *phdrs, *phdr;
>  
>  		phdrs = kzalloc(phdrs_len, GFP_KERNEL);
> @@ -397,22 +400,21 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  			phdr++;
>  		}
>  
> -		tsz = min_t(size_t, buflen, phdrs_offset + phdrs_len - *fpos);
> -		if (copy_to_user(buffer, (char *)phdrs + *fpos - phdrs_offset,
> -				 tsz)) {
> +		tsz = min_t(size_t, buflen, phdrs_offset + phdrs_len - *ppos);
> +		if (copy_to_iter((char *)phdrs + *ppos - phdrs_offset, tsz,
> +				 iter) != tsz) {
>  			kfree(phdrs);
>  			ret = -EFAULT;
>  			goto out;
>  		}
>  		kfree(phdrs);
>  
> -		buffer += tsz;
>  		buflen -= tsz;
> -		*fpos += tsz;
> +		*ppos += tsz;
>  	}
>  
>  	/* ELF note segment. */
> -	if (buflen && *fpos < notes_offset + notes_len) {
> +	if (buflen && *ppos < notes_offset + notes_len) {
>  		struct elf_prstatus prstatus = {};
>  		struct elf_prpsinfo prpsinfo = {
>  			.pr_sname = 'R',
> @@ -447,24 +449,23 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  				  vmcoreinfo_data,
>  				  min(vmcoreinfo_size, notes_len - i));
>  
> -		tsz = min_t(size_t, buflen, notes_offset + notes_len - *fpos);
> -		if (copy_to_user(buffer, notes + *fpos - notes_offset, tsz)) {
> +		tsz = min_t(size_t, buflen, notes_offset + notes_len - *ppos);
> +		if (copy_to_iter(notes + *ppos - notes_offset, tsz, iter) != tsz) {
>  			kfree(notes);
>  			ret = -EFAULT;
>  			goto out;
>  		}
>  		kfree(notes);
>  
> -		buffer += tsz;
>  		buflen -= tsz;
> -		*fpos += tsz;
> +		*ppos += tsz;
>  	}
>  
>  	/*
>  	 * Check to see if our file offset matches with any of
>  	 * the addresses in the elf_phdr on our list.
>  	 */
> -	start = kc_offset_to_vaddr(*fpos - data_offset);
> +	start = kc_offset_to_vaddr(*ppos - data_offset);
>  	if ((tsz = (PAGE_SIZE - (start & ~PAGE_MASK))) > buflen)
>  		tsz = buflen;
>  
> @@ -497,7 +498,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		}
>  
>  		if (!m) {
> -			if (clear_user(buffer, tsz)) {
> +			if (iov_iter_zero(tsz, iter) != tsz) {
>  				ret = -EFAULT;
>  				goto out;
>  			}
> @@ -508,14 +509,14 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		case KCORE_VMALLOC:
>  			vread(buf, (char *)start, tsz);
>  			/* we have to zero-fill user buffer even if no read */
> -			if (copy_to_user(buffer, buf, tsz)) {
> +			if (copy_to_iter(buf, tsz, iter) != tsz) {
>  				ret = -EFAULT;
>  				goto out;
>  			}
>  			break;
>  		case KCORE_USER:
>  			/* User page is handled prior to normal kernel page: */
> -			if (copy_to_user(buffer, (char *)start, tsz)) {
> +			if (copy_to_iter((char *)start, tsz, iter) != tsz) {
>  				ret = -EFAULT;
>  				goto out;
>  			}
> @@ -531,7 +532,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  			 */
>  			if (!page || PageOffline(page) ||
>  			    is_page_hwpoison(page) || !pfn_is_ram(pfn)) {
> -				if (clear_user(buffer, tsz)) {
> +				if (iov_iter_zero(tsz, iter) != tsz) {
>  					ret = -EFAULT;
>  					goto out;
>  				}
> @@ -541,25 +542,24 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		case KCORE_VMEMMAP:
>  		case KCORE_TEXT:
>  			/*
> -			 * We use _copy_to_user() to bypass usermode hardening
> +			 * We use _copy_to_iter() to bypass usermode hardening
>  			 * which would otherwise prevent this operation.
>  			 */
> -			if (_copy_to_user(buffer, (char *)start, tsz)) {
> +			if (_copy_to_iter((char *)start, tsz, iter) != tsz) {
>  				ret = -EFAULT;
>  				goto out;
>  			}
>  			break;
>  		default:
>  			pr_warn_once("Unhandled KCORE type: %d\n", m->type);
> -			if (clear_user(buffer, tsz)) {
> +			if (iov_iter_zero(tsz, iter) != tsz) {
>  				ret = -EFAULT;
>  				goto out;
>  			}
>  		}
>  skip:
>  		buflen -= tsz;
> -		*fpos += tsz;
> -		buffer += tsz;
> +		*ppos += tsz;
>  		start += tsz;
>  		tsz = (buflen > PAGE_SIZE ? PAGE_SIZE : buflen);
>  	}
> @@ -603,7 +603,7 @@ static int release_kcore(struct inode *inode, struct file *file)
>  }
>  
>  static const struct proc_ops kcore_proc_ops = {
> -	.proc_read	= read_kcore,
> +	.proc_read_iter	= read_kcore_iter,
>  	.proc_open	= open_kcore,
>  	.proc_release	= release_kcore,
>  	.proc_lseek	= default_llseek,
> -- 
> 2.39.2
>