Re: [PATCH v2] erofs: accept source file descriptor via fsconfig

[Giuseppe Scrivano <gscrivan@redhat.com>]

Gao Xiang <hsiangkao@linux.alibaba.com> writes:

> On 2026/7/14 08:49, Aleksa Sarai wrote:
>> On 2026-07-13, Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>> thanks for the hints.
>>>
>>> I'll prepare a v3 if you are fine with the version below:
>> No worries, and this seems more reasonable at a first glance.
>> 
>>> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
>>> index 86fa5c6a0c70..72c85cc53085 100644
>>> --- a/fs/erofs/super.c
>>> +++ b/fs/erofs/super.c
>> ...
>>> @@ -437,6 +439,38 @@ static bool erofs_fc_set_dax_mode(struct fs_context *fc, unsigned int mode)
>>>   	return false;
>>>   }
>>>   +static int erofs_fc_parse_source(struct fs_context *fc,
>>> +				 struct fs_parameter *param)
>>> +{
>>> +	struct erofs_sb_info *sbi = fc->s_fs_info;
>>> +
>>> +	if (fc->source || sbi->dif0.file)
>>> +		return invalf(fc, "Multiple sources");
>>> +
>>> +	switch (param->type) {
>>> +	case fs_value_is_string:
>>> +		fc->source = param->string;
>>> +		param->string = NULL;
>>> +		return 0;
>>> +	case fs_value_is_file: {
>>> +		char *buf, *p;
>>> +
>>> +		sbi->dif0.file = get_file(param->file);
>> A very minor nit, but you can actually steal the file reference here
>> with
>> 		sbi->dif0.file = no_free_ptr(param->file);
>> A few other places do this. (You'll also need to change the
>> param->file
>> reference below.)
>> 
>>> +		buf = kmalloc(PATH_MAX, GFP_KERNEL);
>>> +		if (!buf)
>>> +			return -ENOMEM;
>>> +		p = file_path(param->file, buf, PATH_MAX);
>>> +		fc->source = kstrdup(IS_ERR(p) ? "(fd)" : p, GFP_KERNEL);
>> I think that /proc/self/fd/%d would be a more useful name for
>> debugging
>> if file_path() fails (not that it is really possible here AFAICS). But
>> I'm not really too fussed.
>
> Not quite sure if we should get in agreement with the format of this
> one in advance (IOWs, users use source_fd and how fc->source looks like;
> since other fses may follow the same practice if source_fd becomes common
> later) since it's a user-visible field and I believe we shouldn't treat
> this one as a dontcare field as some pseudo fses (since those fses don't
> rely on `fc->source` by design but typically EROFS can rely on.)
>
> I hope Christian and others could share move thought on this part too
> before I land this feature for the next cycle.

would it be better to just return the error from file_path without any
fallback?

Thanks,
Giuseppe