Re: [PATCH v1 1/3] fs: add parameter use_fpos to iterate_dir function

[Stefan Roesch <shr@fb.com>]

On 11/25/21 7:55 AM, Pavel Begunkov wrote:
> On 11/23/21 18:10, Stefan Roesch wrote:
>> This adds the use_fpos parameter to the iterate_dir function.
>> If use_fpos is true it uses the file position in the file
>> structure (existing behavior). If use_fpos is false, it uses
>> the pos in the context structure.
>>
>> This change is required to support getdents in io_uring.
>>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>>   fs/exportfs/expfs.c    |  2 +-
>>   fs/nfsd/nfs4recover.c  |  2 +-
>>   fs/nfsd/vfs.c          |  2 +-
>>   fs/overlayfs/readdir.c |  6 +++---
>>   fs/readdir.c           | 28 ++++++++++++++++++++--------
>>   include/linux/fs.h     |  2 +-
>>   6 files changed, 27 insertions(+), 15 deletions(-)
>>
> [...]
>> diff --git a/fs/readdir.c b/fs/readdir.c
>> index 09e8ed7d4161..8ea5b5f45a78 100644
>> --- a/fs/readdir.c
>> +++ b/fs/readdir.c
>> @@ -21,6 +21,7 @@
>>   #include <linux/unistd.h>
>>   #include <linux/compat.h>
>>   #include <linux/uaccess.h>
>> +#include "internal.h"
>>     #include <asm/unaligned.h>
>>   @@ -36,8 +37,14 @@
>>       unsafe_copy_to_user(dst, src, len, label);        \
>>   } while (0)
>>   -
>> -int iterate_dir(struct file *file, struct dir_context *ctx)
>> +/**
>> + * iterate_dir - iterate over directory
>> + * @file    : pointer to file struct of directory
>> + * @ctx     : pointer to directory ctx structure
>> + * @use_fpos: true : use file offset
>> + *            false: use pos in ctx structure
>> + */
>> +int iterate_dir(struct file *file, struct dir_context *ctx, bool use_fpos)
>>   {
>>       struct inode *inode = file_inode(file);
>>       bool shared = false;
>> @@ -60,12 +67,17 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
>>         res = -ENOENT;
>>       if (!IS_DEADDIR(inode)) {
>> -        ctx->pos = file->f_pos;
>> +        if (use_fpos)
>> +            ctx->pos = file->f_pos;
> 
> One more thing I haven't noticed before, should pos be sanitised
> somehow if passed from the userspace? Do filesystems handle it
> well?
> 

I checked a couple of filesystems and they all check that the pos value is reasonable.

> 
>> +
>>           if (shared)
>>               res = file->f_op->iterate_shared(file, ctx);
>>           else
>>               res = file->f_op->iterate(file, ctx);
>> -        file->f_pos = ctx->pos;
>> +
>> +        if (use_fpos)
>> +            file->f_pos = ctx->pos;
>> +
>>           fsnotify_access(file);
>>           file_accessed(file);
>>       }
> 
>