An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment

georgejoseph — Mon, 16 Oct 2017 21:16:12 GMT

I’m trying to use the ioctl call to an sdcard to search for a filename and then delete it. When the function call returns it has an illegal ram address in the structure. After digging for several days I have collected some information on the problem.

/* MFS part of the software */

MFS_SEARCH_DATA search_data;

MFS_SEARCH_PARAM search;

char filepath[46] = "";// used for the input filename wildcard

char filename_buffer[46]; // returned filename

search.ATTRIBUTE = MFS_SEARCH_LFN;

search.WILDCARD = filepath;

search.SEARCH_DATA_PTR = &search_data;

search.LFN_BUF = filename_buffer;

search.LFN_BUF_LEN = sizeof(filename_buffer);

search. SEARCH_DATA_PTR->ATTRIBUTE = MFS_SEARCH_LFN;

search.SEARCH_DATA_PTR->LFN_BUF = filename_buffer;

search.SEARCH_DATA_PTR->LFN_BUF_LEN = sizeof(filename_buffer);

// Check to see if this file exists. If so then delete it before creating a new one

sprintf(filepath, "snapshot%d_*.bin", file_index.intervention_snapshot_file); // wildcard filename

error_code = ioctl(a_fd, IO_IOCTL_FIND_FIRST_FILE, &search);

if(error_code == MFS_NO_ERROR)

{

sprintf(int_snapshot_file_name, "%s", search.SEARCH_DATA_PTR->LFN_BUF);

}

while(error_code == MFS_NO_ERROR)

{

error_code = ioctl(a_fd, IO_IOCTL_DELETE_FILE, int_snapshot_file_name);

error_code = ioctl(a_fd, IO_IOCTL_FIND_NEXT_FILE,search.SEARCH_DATA_PTR); // Check if there are more

if(error_code == MFS_NO_ERROR)

{

sprintf(int_snapshot_file_name, "%s", search.SEARCH_DATA_PTR->LFN_BUF);

}

A look at the addresses of the structure before and after the function call:

Just before the call to error_code = ioctl(a_fd, IO_IOCTL_FIND_FIRST_FILE, &search);

(Had to create variables to read addresses because the Expressions window was giving a different value than when I did a mouse over). These values are from MFS_SEARCH_PARAM.

a = (uint32_t)&search.SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.WILDCARD;

b = (uint32_t)&search.SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.ATTR_ONE_MASK;

c = (uint32_t)&search.SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.ATTR_ZERO_MASK;

d = (uint32_t)&search.SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.DIR_CHAIN_LOC;

e = (uint32_t)&search.SEARCH_DATA_PTR->LFN_BUF; = 0x20012fc6

After stepping into the file mfs_find.c and function _mfs_error MFS_find_init( MFS_DRIVE_STRUCT_PTR drive_ptr,

MFS_SEARCH_PARAM_PTR sp_ptr,

MFS_SEARCH_DATA_PTR sd_ptr):

The sp_ptr pointer is pointing to the same address as "search" from above.

a = (uint32_t)&sp_ptr->SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.WILDCARD;

b = (uint32_t)&sp_ptr->SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.ATTR_ONE_MASK;

c = (uint32_t)&sp_ptr->SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.ATTR_ZERO_MASK;

d = (uint32_t)&sp_ptr->SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.DIR_CHAIN_LOC;

e = (uint32_t)&sp_ptr->SEARCH_DATA_PTR->LFN_BUF;

In sp_ptr->SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.DIR_CHAIN_LOC you can see that address of this element of the structure MFS_SEARCH_PARAM is two bytes different than in search.SEARCH_DATA_PTR->INTERNAL_SEARCH_DATA.DIR_CHAIN_LOC (0x20012f9e vs 0x2001fa0)

When the line sd_ptr->LFN_BUF = sp_ptr->LFN_BUF; is reached (in MFS_find_init()) it copies the filename to 0x20012fa0 and when it returns to the calling function the variable “search” looks for the filename in 0x20012f9e resulting in a shift of the contents of LFN_BUF.

Any ideas why this is happening?

Re: An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment

georgejoseph — Wed, 18 Oct 2017 13:56:43 GMT

Solution: I recently split my .h files into prototype.h and struct.h because I was having problems with circular references. In my new struct.h file I added an #include for another .h file which contained a #pragma pack(1) directive. This would have been ok except there was not corresponding #pragma pack() to turn off the packing so my new .h file inherited the pack command and the MFS_SEARCH_PARAM search and MFS_SEARCH_DATA search_data were packed to a one byte alignment. Unfortunately the mfs.a library file had the default alignment, so this is what caused the alignment problem. I changed to using the #pragma pack(push, 1) coupled with a #pragma pack(pop). This cleared up the alignment problem.

Re: An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment

danielchen — Thu, 19 Oct 2017 02:59:42 GMT

Thank you for your sharing.

Regards

Daniel

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An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment

Re: An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment

Re: An MFS function call ioctl using IO_IOCTL_FIND_FIRST_FILE results in structure misalignment