[PATCH 1/1] fs: fix smh_fs_read_at()

[Heinrich Schuchardt]

On 6/2/23 18:43, Sean Anderson wrote:
> On 6/2/23 12:21, Heinrich Schuchardt wrote:
>> On 6/2/23 17:56, Sean Anderson wrote:
>>> On 6/2/23 04:48, Heinrich Schuchardt wrote:
>>>>
>>>>
>>>> On 5/18/23 17:17, Sean Anderson wrote:
>>>>> On 5/17/23 06:23, Heinrich Schuchardt wrote:
>>>>>> The return value of smh_flen() is written to size and not to ret. But ret
>>>>>> is checked. We can avoid calling smh_flen() by setting maxsize to LONG_MAX
>>>>>> if it is not set yet.
>>>>>>
>>>>>> Check input parameters.
>>>>>>
>>>>>> Fixes: f676b45151c3 ("fs: Add semihosting filesystem")
>>>>>> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
>>>>>> ---
>>>>>>     fs/semihostingfs.c | 14 +++++---------
>>>>>>     1 file changed, 5 insertions(+), 9 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/semihostingfs.c b/fs/semihostingfs.c
>>>>>> index 96eb3349a2..8a7d4da884 100644
>>>>>> --- a/fs/semihostingfs.c
>>>>>> +++ b/fs/semihostingfs.c
>>>>>> @@ -25,6 +25,9 @@ static int smh_fs_read_at(const char *filename, loff_t pos, void *buffer,
>>>>>>     {
>>>>>>         long fd, size, ret;
>>>>>>     +    if (pos > LONG_MAX || maxsize > LONG_MAX)
>>>>>
>>>>> Should be ULONG_MAX. The type should really be ulong but isn't.
>>>>>
>>>>>> +        return -EINVAL;
>>>>>> +
>>>>>>         fd = smh_open(filename, MODE_READ | MODE_BINARY);
>>>>>>         if (fd < 0)
>>>>>>             return fd;
>>>>>> @@ -33,15 +36,8 @@ static int smh_fs_read_at(const char *filename, loff_t pos, void *buffer,
>>>>>>             smh_close(fd);
>>>>>>             return ret;
>>>>>>         }
>>>>>> -    if (!maxsize) {
>>>>>> -        size = smh_flen(fd);
>>>>>> -        if (ret < 0) {
>>>>>> -            smh_close(fd);
>>>>>> -            return size;
>>>>>> -        }
>>>>>> -
>>>>>> -        maxsize = size;
>>>>>> -    }
>>>>>> +    if (!maxsize)
>>>>>> +        maxsize = LONG_MAX;
>>>>>
>>>>> Same here.
>>>>>
>>>>>>           size = smh_read(fd, buffer, maxsize);
>>>>
>>>> Thanks for reviewing.
>>>>
>>>> Just changing to ULONG_MAX will not work, because smh_read() returns errors as negative numbers.
>>>>
>>>> We would need to change the interface of smh_read to:
>>>>
>>>> int smh_read(long fd, void *memp, unsigned long *len)
>>>>
>>>> Then we could return the error code and the number of bytes read separately.
>>>
>>> Sounds reasonable.
>>>
>>>> https://cas5-0-urlprotect.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fdeveloper.arm.com%2fdocumentation%2fdui0471%2fm%2fwhat%2dis%2dsemihosting%2d%2fsys%2dread%2d%2d0x06%2d&umid=985b6a56-4e08-43b2-84c0-f256a70807de&auth=d807158c60b7d2502abde8a2fc01f40662980862-076ac538dd94fd766377877ad7a42d77df41830f
>>>> describes that SYS_READ may return less bytes than requested. This does not signify that EOF has been reached but may simply reflect the behavior of the underlying operating system (cf. 'man 2 read').
>>>>> So shouldn't we loop here until all bytes are read?
>>>
>>> We return the number of bytes read. It's up to the caller to loop if they want.
>>>
>>> IMO for efficiency, the host should handle short reads to avoid multiple semihosting calls.
>>
>> Have a look at the QEMU code. host_read() calls read() once. So if the underlying host file system does not read all bytes in the first invocation (which is allowable), SYS_READ will not do so either.
> 
> Yes, I know that some hosts do not do this, but they should for efficiency.
> 
>> U-Boot's do_load() calls _fs_read() only once. It expects the semihosting filesystem to return the complete file.
>>
>> So there seems to be a gap in U-Boot's semihosting code.
> 
> Isn't that what the fs_read's actread parameter is for?

All usages of fs_read() expect that actread returns the size of the 
complete file, e.g.

- sysboot_read_file()
- do_load()
- splash_load_fs()

Other U-Boot file systems like FAT, EXT4 conform to this.

Unfortunately struct fstype_info is not documented at all and the 
documentation of fs_read() in include/fs.h does not point this out.

Best regards

Heinrich