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shoehorned CRC32 thanks to Clang optimization improvements
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@majbthrd
majbthrd committed Oct 21, 2018
1 parent 83b6ea5 commit 0bb7b49
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12 changes: 3 additions & 9 deletions README.md
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Expand Up @@ -11,13 +11,7 @@ It is a much more space efficient alternative to the 4kB Atmel/Microchip [AN_423

Downloading can be accomplished with the existing [dfu-util](http://dfu-util.sourceforge.net/) utilities.

Downloading a raw binary file looks like this:

```
dfu-util -D write.bin
```

or by using the provided dx1elf2dfu utility, one can create a .dfu file to be downloaded:
Using the provided dx1elf2dfu utility, one can create a .dfu file; that file can then be downloaded like so:

```
dfu-util -D write.dfu
Expand All @@ -27,13 +21,13 @@ dfu-util -D write.dfu

The linker memory map of the user application must be modified to have an origin at 0x0000_0400 rather than at 0x0000_0000. This bootloader resides at 0x0000_0000.

When booting, the bootloader checks whether a GPIO pin (nominally PA15) is connected to ground. If so, it runs the bootloader instead of the user application.
When booting, the bootloader checks whether a GPIO pin (nominally PA15) is connected to ground. It also computes a CRC32 of the user application. If the user application is unprogrammed or corrupted, the CRC32 check should fall. If the CRC32 check fails or the GPIO pin is grounded, it runs the bootloader instead of the user application.

When branching to the user application, the bootloader includes functionality to update the [VTOR (Vector Table Offset Register)](http://infocenter.arm.com/help/topic/com.arm.doc.dui0662a/Ciheijba.html) and update the stack pointer to suit the value in the user application's vector table.

## Requirements for compiling

[Rowley Crossworks for ARM](http://www.rowley.co.uk/arm/) is presently needed to compile this code. With Crossworks for ARM v4.1.1, using the Clang 5.0.1 compiler produces an 1010 byte image. The more mainstream GCC does not appear to be optimized enough to produce an image that comes anywhere close to fitting into 1024 bytes.
[Rowley Crossworks for ARM](http://www.rowley.co.uk/arm/) is presently needed to compile this code. With Crossworks for ARM v4.3.0, using the Clang 7.0.0 compiler produces a 1014 byte image. The more mainstream GCC does not appear to be optimized enough to produce an image that comes anywhere close to fitting into 1024 bytes.

There is no dependency in the code on the [Rowley Crossworks for ARM](http://www.rowley.co.uk/arm/) toolchain per se, but at this time I am not aware of any other ready-to-use Clang ARM cross-compiler package that I can readily point users to.

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18 changes: 16 additions & 2 deletions bootloader.c
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Expand Up @@ -242,9 +242,23 @@ void bootloader(void)
PORT->Group[0].PINCFG[15].reg = PORT_PINCFG_PULLEN | PORT_PINCFG_INEN;
PORT->Group[0].OUTSET.reg = (1UL << 15);

if (PORT->Group[0].IN.reg & (1UL << 15))
return; /* pin not grounded, so run user app */
PAC1->WPCLR.reg = 2; /* clear DSU */

DSU->ADDR.reg = 0x400; /* start CRC check at beginning of user app */
DSU->LENGTH.reg = *(volatile uint32_t *)0x410; /* use length encoded into unused vector address in user app */

/* ask DSU to compute CRC */
DSU->DATA.reg = 0xFFFFFFFF;
DSU->CTRL.bit.CRC = 1;
while (!DSU->STATUSA.bit.DONE);

if (!(PORT->Group[0].IN.reg & (1UL << 15)))
goto run_bootloader; /* pin grounded, so run bootloader */

if (0 == DSU->DATA.reg)
return; /* CRC passes, so run user app */

run_bootloader:
/*
configure oscillator for crystal-free USB operation
*/
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9 changes: 8 additions & 1 deletion dx1elf2dfu/README.md
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@@ -1,11 +1,18 @@
dx1elf2dfu
==========

This tool is a possible aid for developers who use the Dx1bootloader and want to generate a DFU image directly from a Dx1 ELF object file.
This tool is for users of the Dx1bootloader to generate a DFU image directly from a SAMD11/SAMD21 ELF object file.

This DFU image output includes a CRC32 calculation that is stored inside the user application; this is checked by the bootloader to verify the user application's integrity.

## Sample Usage

```
dx1elf2dfu myapp.elf myapp.dfu
```

## Theory of Operation

The Cortex-M0 vector table has nine unused 32-bit entries marked 'Reserved' that only serve as wasted flash space. By using two of these entries to store the user application length and its CRC32, the dx1elf2dfu utility can communicate this information to the bootloader without consuming additional space.

By storing the user application length, the bootloader will only compute the CRC32 over a prescribed portion of the flash. This frees the user application, if it wishes, to store and re-write data in upper portions of the flash without impacting the CRC32 protection.
209 changes: 186 additions & 23 deletions dx1elf2dfu/dx1elf2dfu.c
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Expand Up @@ -6,6 +6,10 @@
https://github.com/majbthrd/SAMDx1-USB-DFU-Bootloader
and thus expects the input ELF to not use the first 1024 bytes.
The bootloader expects an application length and CRC32 to be stored
without the user application (using unused vector table entries). This
tool calculates these quantities and inserts them into the output DFU.
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
Expand Down Expand Up @@ -33,6 +37,10 @@
#define USB_VENDOR_ID 0x1209
#define USB_PRODUCT_ID 0x2003

static const uint32_t origin_addr = 0x400; /* origin of the application (first address available after the bootloader) */
static const uint32_t app_len_offset = 0x10; /* reserved application vector where the application size is stored */
static const uint32_t app_crc_offset = 0x14; /* reserved application vector where the CRC value is stored */

typedef struct Elf32_Ehdr
{
uint8_t e_ident[16];
Expand Down Expand Up @@ -241,7 +249,7 @@ static struct memory_blob *find_blob(uint32_t address, uint32_t count, struct me
}

addition = malloc(sizeof(struct memory_blob));
memset(addition, 0, sizeof(struct memory_blob));
memset(addition, 0xFF, sizeof(struct memory_blob));

addition->data = malloc(count);
addition->address = address;
Expand Down Expand Up @@ -324,6 +332,26 @@ static const uint32_t crc32_table[256] =
0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D,
};

static const uint8_t high_lookup[256] =
{
0, 65, 195, 130, 134, 199, 69, 4, 77, 12, 142, 207, 203, 138, 8, 73,
154, 219, 89, 24, 28, 93, 223, 158, 215, 150, 20, 85, 81, 16, 146, 211,
117, 52, 182, 247, 243, 178, 48, 113, 56, 121, 251, 186, 190, 255, 125, 60,
239, 174, 44, 109, 105, 40, 170, 235, 162, 227, 97, 32, 36, 101, 231, 166,
234, 171, 41, 104, 108, 45, 175, 238, 167, 230, 100, 37, 33, 96, 226, 163,
112, 49, 179, 242, 246, 183, 53, 116, 61, 124, 254, 191, 187, 250, 120, 57,
159, 222, 92, 29, 25, 88, 218, 155, 210, 147, 17, 80, 84, 21, 151, 214,
5, 68, 198, 135, 131, 194, 64, 1, 72, 9, 139, 202, 206, 143, 13, 76,
149, 212, 86, 23, 19, 82, 208, 145, 216, 153, 27, 90, 94, 31, 157, 220,
15, 78, 204, 141, 137, 200, 74, 11, 66, 3, 129, 192, 196, 133, 7, 70,
224, 161, 35, 98, 102, 39, 165, 228, 173, 236, 110, 47, 43, 106, 232, 169,
122, 59, 185, 248, 252, 189, 63, 126, 55, 118, 244, 181, 177, 240, 114, 51,
127, 62, 188, 253, 249, 184, 58, 123, 50, 115, 241, 176, 180, 245, 119, 54,
229, 164, 38, 103, 99, 34, 160, 225, 168, 233, 107, 42, 46, 111, 237, 172,
10, 75, 201, 136, 140, 205, 79, 14, 71, 6, 132, 197, 193, 128, 2, 67,
144, 209, 83, 18, 22, 87, 213, 148, 221, 156, 30, 95, 91, 26, 152, 217,
};

static uint32_t crc32_calc(uint32_t crc, uint8_t *buffer, uint32_t length)
{
while (length--)
Expand All @@ -332,6 +360,46 @@ static uint32_t crc32_calc(uint32_t crc, uint8_t *buffer, uint32_t length)
return crc;
}

static uint32_t reverse_crc32_calc(uint32_t crc, const uint8_t *buffer, uint32_t length)
{
const uint8_t *data = buffer + length;
uint8_t high_byte;

while (length--)
{
data--;
high_byte = (uint8_t)(crc >> 24);
crc ^= crc32_table[high_lookup[high_byte]];
crc <<= 8;
crc += high_lookup[high_byte] ^ *data;
}

return crc;
}

static uint32_t calc_span(uint32_t prior_crc, uint32_t post_crc)
{
int index;
uint8_t byte;
uint32_t span, table;

for (index = 3; index >= 0; index--)
{
table = (table << 8) | high_lookup[post_crc >> 24];
post_crc = (post_crc ^ crc32_table[table & 0xFF]) << 8;
}
for (index = 0; index < 4; index++)
{
byte = (uint8_t)(prior_crc ^ table);
prior_crc = (prior_crc >> 8) ^ crc32_table[table & 0xFF];
span >>= 8;
span |= (uint32_t)byte << 24;
table >>= 8;
}

return span;
}

int main(int argc, char *argv[])
{
FILE *elffp;
Expand All @@ -340,8 +408,10 @@ int main(int argc, char *argv[])
Elf32_Phdr *ph;
Elf32_Shdr sh;
struct memory_blob *blob, *pm_list;
uint32_t phy_addr, origin_addr, crc32, stuff_size;
uint32_t phy_addr, dfu_crc32, stuff_size, max_offset;
uint8_t scratchpad[64 /* sized to be at least as large as the DFU suffix */];
uint32_t pre_crc, post_crc, span;
uint8_t *binary;

if (argc < 3)
{
Expand Down Expand Up @@ -407,7 +477,7 @@ int main(int argc, char *argv[])
fclose(elffp);

/*
write blob list to raw file
sanity check blob list
*/

blob = pm_list;
Expand All @@ -418,43 +488,136 @@ int main(int argc, char *argv[])
return -1;
}

origin_addr = 0x400; /* this is the first address available after the bootloader */
crc32 = 0xFFFFFFFF;

if (blob->address < origin_addr)
{
printf("ERROR: provided ELF intrudes into bootloader space; DFU cannot be created\n");
return -1;
}

if (blob->address != origin_addr)
{
printf("ERROR: provided ELF must start at 0x%x; DFU cannot be created\n", origin_addr);
return -1;
}

dfufp = fopen(argv[2], "wb");
if (!dfufp)
{
printf("ERROR: unable to open file <%s> for writing\n", argv[2]);
return -1;
}

blob = pm_list; phy_addr = origin_addr;

while (blob)
{
phy_addr = blob->address;
stuff_size = blob->address - phy_addr;

while (origin_addr < phy_addr)
if (stuff_size)
{
memset(scratchpad, 0xFF, sizeof(scratchpad));
stuff_size = phy_addr - origin_addr;
if (stuff_size > sizeof(scratchpad))
stuff_size = sizeof(scratchpad);
crc32 = crc32_calc(crc32, scratchpad, stuff_size);
fwrite(scratchpad, stuff_size, 1, dfufp);
origin_addr += stuff_size;
/* there is a gap, so create a blank region here */
find_blob(phy_addr, stuff_size, &pm_list);
}

crc32 = crc32_calc(crc32, blob->data, blob->count);
fwrite(blob->data, blob->count, 1, dfufp);
origin_addr += blob->count;
/* figure the best-case starting point of the next blob */
phy_addr = blob->address + blob->count;

/* advance to next blob */
blob = blob->next;
}

max_offset = phy_addr - origin_addr;

/* check if program image ends on a 32-bit boundary */
if (phy_addr & 0x3)
{
stuff_size = 4 - (phy_addr & 0x3);
max_offset += stuff_size;
/* create blank region at end to pad out to whole 32-bits (as the CRC will require this) */
find_blob(phy_addr, stuff_size, &pm_list);
}

/*
The upcoming CRC calculations are artificially complicated with using blobs, so we
take the approach of moving all the blobs into a malloc-ed linear region of memory.
*/

binary = malloc(max_offset);

while (pm_list)
{
blob = pm_list;
memcpy(binary + blob->address - origin_addr, blob->data, blob->count);
pm_list = pm_list->next;
free(blob);
}

/*
as a sanity check, we check what the value is of the vector entry we over-write with the length
*/

stuff_size = (uint32_t)binary[app_len_offset + 3] << 24;
stuff_size |= (uint32_t)binary[app_len_offset + 2] << 16;
stuff_size |= (uint32_t)binary[app_len_offset + 1] << 8;
stuff_size |= (uint32_t)binary[app_len_offset + 0] << 0;

if (stuff_size)
{
printf("WARNING: overwriting 0x%x at 0x%x with length value (%d)\n", stuff_size, origin_addr + app_len_offset, max_offset);
}

/* store app length within application itself */
binary[app_len_offset + 0] = (uint8_t)(max_offset >> 0);
binary[app_len_offset + 1] = (uint8_t)(max_offset >> 8);
binary[app_len_offset + 2] = (uint8_t)(max_offset >> 16);
binary[app_len_offset + 3] = (uint8_t)(max_offset >> 24);

/*
perform the calculation to determine what 32-bit value to write
in order to cause the overall CRC32 to calculate to be zero
*/

pre_crc = crc32_calc(0xFFFFFFFF /* starting CRC value */, binary, app_crc_offset);
post_crc = reverse_crc32_calc(0 /* desired end value */, binary + app_crc_offset + 4, max_offset - (app_crc_offset + 4));
span = calc_span(pre_crc, post_crc);

/*
as a sanity check, we check what the value is of the vector entry we over-write with the CRC32
*/

stuff_size = (uint32_t)binary[app_crc_offset + 3] << 24;
stuff_size |= (uint32_t)binary[app_crc_offset + 2] << 16;
stuff_size |= (uint32_t)binary[app_crc_offset + 1] << 8;
stuff_size |= (uint32_t)binary[app_crc_offset + 0] << 0;

if (stuff_size)
{
printf("WARNING: overwriting 0x%x at 0x%x with CRC value (%d)\n", stuff_size, origin_addr + app_crc_offset, span);
}

/* store app CRC within application itself */
binary[app_crc_offset + 0] = (uint8_t)(span >> 0);
binary[app_crc_offset + 1] = (uint8_t)(span >> 8);
binary[app_crc_offset + 2] = (uint8_t)(span >> 16);
binary[app_crc_offset + 3] = (uint8_t)(span >> 24);

stuff_size = crc32_calc(0xFFFFFFFF, binary, max_offset);

if (stuff_size)
{
printf("ERROR: something has gone wrong with the CRC calculation; value was 0x%x\n", stuff_size);
return -1;
}

/* start the DFU CRC32 calculation */
dfu_crc32 = crc32_calc(0xFFFFFFFF, binary, max_offset);

/* write the tweaked image into the DFU file */
fwrite(binary, max_offset, 1, dfufp);

/* free remaining malloc-ed memory */
free(binary);

/*
append DFU standard suffix
*/
Expand All @@ -474,12 +637,12 @@ int main(int argc, char *argv[])
scratchpad[i++] = 16; // bLength

/* the CRC-32 has now been calculated over the entire file, save for the CRC field itself */
crc32 = crc32_calc(crc32, scratchpad, i);
dfu_crc32 = crc32_calc(dfu_crc32, scratchpad, i);

scratchpad[i++] = (uint8_t)(crc32 >> 0);
scratchpad[i++] = (uint8_t)(crc32 >> 8);
scratchpad[i++] = (uint8_t)(crc32 >> 16);
scratchpad[i++] = (uint8_t)(crc32 >> 24);
scratchpad[i++] = (uint8_t)(dfu_crc32 >> 0);
scratchpad[i++] = (uint8_t)(dfu_crc32 >> 8);
scratchpad[i++] = (uint8_t)(dfu_crc32 >> 16);
scratchpad[i++] = (uint8_t)(dfu_crc32 >> 24);

fwrite(scratchpad, i, 1, dfufp);

Expand Down
2 changes: 1 addition & 1 deletion usb_descriptors.c
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Expand Up @@ -45,7 +45,7 @@ usb_device_descriptor_t usb_device_descriptor __attribute__ ((aligned (4))) = /*
.bMaxPacketSize0 = 64,
.idVendor = 0x1209,
.idProduct = 0x2003,
.bcdDevice = 0x0101,
.bcdDevice = 0x0102,

.iManufacturer = USB_STR_ZERO,
.iProduct = USB_STR_ZERO,
Expand Down

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