linux-kernel - Re: [PATCH] CIFS: Fix bug which the return value by asynchronous read is error

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Date:   Thu, 19 Mar 2020 10:50:16 +0800
From:   Yilu Lin <linyilu@...wei.com>
To:     ronnie sahlberg <ronniesahlberg@...il.com>
CC:     linux-cifs <linux-cifs@...r.kernel.org>,
        Steve French <sfrench@...ba.org>,
        "samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
        LKML <linux-kernel@...r.kernel.org>, <alex.chen@...wei.com>
Subject: Re: [PATCH] CIFS: Fix bug which the return value by asynchronous read
 is error

Hi,
The bug is trigerred by the following test program.
First, run the command to create one file on the share directory。COMMAND：dd if=/dev/zero of=/home/temp/cifs/sys.zero bs=512 count=1000 oflag=direct
And then run the c program by the command: ./pread /home/temp/cifs/sys.zero
The program will return "pread -22 bytes".However, the expected result is "pread -1 bytes".

C program code is showed below:

    #include<stdio.h>
    #include<stdlib.h>
    #include<unistd.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #define __USE_GNU 1
    #include <fcntl.h>
    int main(int argc, char *argv[])
    {
            int fd;
            ssize_t len;
            int ret;
            int size = 512;
            char *file=argv[1];

            unsigned char *buf2;
            ret = posix_memalign((void **)&buf2, 512, size);
            if (ret) {
                    printf("malloc err!\n");
                    return 0;
            }

            fd = open(file, O_RDWR | O_DIRECT);
            if(fd == -1) {
                    printf("open error!\n");
                    free(buf2);
                    return 0;
            }

            len = pread(fd, buf2, 504, 511992);
            printf("pread %d bytes\n", len);
            free(buf2);
            close(fd);
            return 0;
    }

regards
Yilu Lin

在 2020/3/18 12:47, ronnie sahlberg 写道:
> Hi Yilu,
> 
> I think your reasoning makes sense.
> Do you have a small reproducer for this? A small C program that triggers this?
> 
> I am asking because if you do we would like to add it to our buildbot
> to make  sure we don't get regressions.
> 
> 
> regards
> ronnie sahlberg
> 
> On Wed, Mar 18, 2020 at 1:59 PM Yilu Lin <linyilu@...wei.com> wrote:
>>
>> This patch is used to fix the bug in collect_uncached_read_data()
>> that rc is automatically converted from a signed number to an
>> unsigned number when the CIFS asynchronous read fails.
>> It will cause ctx->rc is error.
>>
>> Example:
>> Share a directory and create a file on the Windows OS.
>> Mount the directory to the Linux OS using CIFS.
>> On the CIFS client of the Linux OS, invoke the pread interface to
>> deliver the read request.
>>
>> The size of the read length plus offset of the read request is greater
>> than the maximum file size.
>>
>> In this case, the CIFS server on the Windows OS returns a failure
>> message (for example, the return value of
>> smb2.nt_status is STATUS_INVALID_PARAMETER).
>>
>> After receiving the response message, the CIFS client parses
>> smb2.nt_status to STATUS_INVALID_PARAMETER
>> and converts it to the Linux error code (rdata->result=-22).
>>
>> Then the CIFS client invokes the collect_uncached_read_data function to
>> assign the value of rdata->result to rc, that is, rc=rdata->result=-22.
>>
>> The type of the ctx->total_len variable is unsigned integer,
>> the type of the rc variable is integer, and the type of
>> the ctx->rc variable is ssize_t.
>>
>> Therefore, during the ternary operation, the value of rc is
>> automatically converted to an unsigned number. The final result is
>> ctx->rc=4294967274. However, the expected result is ctx->rc=-22.
>>
>> Signed-off-by: Yilu Lin <linyilu@...wei.com>
>> ---
>>  fs/cifs/file.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
>> index 022029a5d..ff4ac244c 100644
>> --- a/fs/cifs/file.c
>> +++ b/fs/cifs/file.c
>> @@ -3323,7 +3323,7 @@ again:
>>         if (rc == -ENODATA)
>>                 rc = 0;
>>
>> -       ctx->rc = (rc == 0) ? ctx->total_len : rc;
>> +       ctx->rc = (rc == 0) ? (ssize_t)ctx->total_len : rc;
>>
>>         mutex_unlock(&ctx->aio_mutex);
>>
>> --
>> 2.19.1
>>
>>
> 
> .
>