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fw-cfg: support writeable blobs 

Useful to send guest data back to QEMU.

Changes from Laszlo Ersek <lersek@redhat.com>:
- rebase the patch from Michael Tsirkin's original postings at [1] and [2]
  to the following patches:
  - loader: Allow a custom AddressSpace when loading ROMs
  - loader: Add AddressSpace loading support to uImages
  - loader: fix handling of custom address spaces when adding ROM blobs
- reject such writes immediately that would exceed the end of the array,
  rather than performing a partial write before setting the error bit: see
  the (len != dma.length) condition
- document the write interface

[1] http://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg04968.html
[2] http://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02735.html

Cc: "Gabriel L. Somlo" <somlo@cmu.edu>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Shannon Zhao <zhaoshenglong@huawei.com>
Cc: qemu-arm@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
This commit is contained in:
Michael S. Tsirkin 2017-01-12 19:24:14 +01:00
parent c471ad0e9b
commit baf2d5bfba
8 changed files with 75 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
docs/specs/fw_cfg.txt
View File

@ -33,6 +33,10 @@ the selector value is between 0x4000-0x7fff or 0xc000-0xffff.
NOTE: As of QEMU v2.4, writes to the fw_cfg data register are no NOTE: As of QEMU v2.4, writes to the fw_cfg data register are no
longer supported, and will be ignored (treated as no-ops)! longer supported, and will be ignored (treated as no-ops)!
NOTE: As of QEMU v2.9, writes are reinstated, but only through the DMA
interface (see below). Furthermore, writeability of any specific item is
governed independently of Bit14 in the selector key value.
Bit15 of the selector register indicates whether the configuration Bit15 of the selector register indicates whether the configuration
setting is architecture specific. A value of 0 means the item is a setting is architecture specific. A value of 0 means the item is a
generic configuration item. A value of 1 means the item is specific generic configuration item. A value of 1 means the item is specific
@ -43,7 +47,7 @@ value between 0x8000-0xffff.
== Data Register == == Data Register ==
* Read/Write (writes ignored as of QEMU v2.4) * Read/Write (writes ignored as of QEMU v2.4, but see the DMA interface)
* Location: platform dependent (IOport [*] or MMIO) * Location: platform dependent (IOport [*] or MMIO)
* Width: 8-bit (if IOport), 8/16/32/64-bit (if MMIO) * Width: 8-bit (if IOport), 8/16/32/64-bit (if MMIO)
* Endianness: string-preserving * Endianness: string-preserving
@ -134,8 +138,8 @@ struct FWCfgFile { /* an individual file entry, 64 bytes total */
=== All Other Data Items === === All Other Data Items ===
Please consult the QEMU source for the most up-to-date and authoritative Please consult the QEMU source for the most up-to-date and authoritative list
list of selector keys and their respective items' purpose and format. of selector keys and their respective items' purpose, format and writeability.
=== Ranges === === Ranges ===
@ -144,9 +148,11 @@ items, and up to 0x4000 architecturally specific ones.
Selector Reg. Range Usage Selector Reg. Range Usage
--------------- ----------- --------------- -----------
0x0000 - 0x3fff Generic (0x0000 - 0x3fff, RO) 0x0000 - 0x3fff Generic (0x0000 - 0x3fff, generally RO, possibly RW through
the DMA interface in QEMU v2.9+)
0x4000 - 0x7fff Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+) 0x4000 - 0x7fff Generic (0x0000 - 0x3fff, RW, ignored in QEMU v2.4+)
0x8000 - 0xbfff Arch. Specific (0x0000 - 0x3fff, RO) 0x8000 - 0xbfff Arch. Specific (0x0000 - 0x3fff, generally RO, possibly RW
through the DMA interface in QEMU v2.9+)
0xc000 - 0xffff Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+) 0xc000 - 0xffff Arch. Specific (0x0000 - 0x3fff, RW, ignored in v2.4+)
In practice, the number of allowed firmware configuration items is given In practice, the number of allowed firmware configuration items is given
@ -182,6 +188,7 @@ The "control" field has the following bits:
- Bit 1: Read - Bit 1: Read
- Bit 2: Skip - Bit 2: Skip
- Bit 3: Select. The upper 16 bits are the selected index. - Bit 3: Select. The upper 16 bits are the selected index.
- Bit 4: Write
When an operation is triggered, if the "control" field has bit 3 set, the When an operation is triggered, if the "control" field has bit 3 set, the
upper 16 bits are interpreted as an index of a firmware configuration item. upper 16 bits are interpreted as an index of a firmware configuration item.
@ -191,8 +198,17 @@ If the "control" field has bit 1 set, a read operation will be performed.
"length" bytes for the current selector and offset will be copied into the "length" bytes for the current selector and offset will be copied into the
physical RAM address specified by the "address" field. physical RAM address specified by the "address" field.
If the "control" field has bit 2 set (and not bit 1), a skip operation will be If the "control" field has bit 4 set (and not bit 1), a write operation will be
performed. The offset for the current selector will be advanced "length" bytes. performed. "length" bytes will be copied from the physical RAM address
specified by the "address" field to the current selector and offset. QEMU
prevents starting or finishing the write beyond the end of the item associated
with the current selector (i.e., the item cannot be resized). Truncated writes
are dropped entirely. Writes to read-only items are also rejected. All of these
write errors set bit 0 (the error bit) in the "control" field.
If the "control" field has bit 2 set (and neither bit 1 nor bit 4), a skip
operation will be performed. The offset for the current selector will be
advanced "length" bytes.
To check the result, read the "control" field: To check the result, read the "control" field:
error bit set -> something went wrong. error bit set -> something went wrong.
@ -234,3 +250,5 @@ Prefix "opt/org.qemu/" is reserved for QEMU itself.
Use of names not beginning with "opt/" is potentially dangerous and Use of names not beginning with "opt/" is potentially dangerous and
entirely unsupported. QEMU will warn if you try. entirely unsupported. QEMU will warn if you try.
All externally provided fw_cfg items are read-only to the guest.

2
hw/arm/virt-acpi-build.c
View File

@ -818,7 +818,7 @@ static MemoryRegion *acpi_add_rom_blob(AcpiBuildState *build_state,
uint64_t max_size) uint64_t max_size)
{ {
return rom_add_blob(name, blob->data, acpi_data_len(blob), max_size, -1, return rom_add_blob(name, blob->data, acpi_data_len(blob), max_size, -1,
name, virt_acpi_build_update, build_state, NULL); name, virt_acpi_build_update, build_state, NULL, true);
} }
static const VMStateDescription vmstate_virt_acpi_build = { static const VMStateDescription vmstate_virt_acpi_build = {

18
hw/core/loader.c
View File

@ -853,7 +853,7 @@ static void fw_cfg_resized(const char *id, uint64_t length, void *host)
} }
} }
static void *rom_set_mr(Rom *rom, Object *owner, const char *name) static void *rom_set_mr(Rom *rom, Object *owner, const char *name, bool ro)
{ {
void *data; void *data;
@ -862,7 +862,7 @@ static void *rom_set_mr(Rom *rom, Object *owner, const char *name)
rom->datasize, rom->romsize, rom->datasize, rom->romsize,
fw_cfg_resized, fw_cfg_resized,
&error_fatal); &error_fatal);
memory_region_set_readonly(rom->mr, true); memory_region_set_readonly(rom->mr, ro);
vmstate_register_ram_global(rom->mr); vmstate_register_ram_global(rom->mr);
data = memory_region_get_ram_ptr(rom->mr); data = memory_region_get_ram_ptr(rom->mr);
@ -942,7 +942,7 @@ int rom_add_file(const char *file, const char *fw_dir,
snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name); snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name);
if ((!option_rom || mc->option_rom_has_mr) && mc->rom_file_has_mr) { if ((!option_rom || mc->option_rom_has_mr) && mc->rom_file_has_mr) {
data = rom_set_mr(rom, OBJECT(fw_cfg), devpath); data = rom_set_mr(rom, OBJECT(fw_cfg), devpath, true);
} else { } else {
data = rom->data; data = rom->data;
} }
@ -979,7 +979,7 @@ err:
MemoryRegion *rom_add_blob(const char *name, const void *blob, size_t len, MemoryRegion *rom_add_blob(const char *name, const void *blob, size_t len,
size_t max_len, hwaddr addr, const char *fw_file_name, size_t max_len, hwaddr addr, const char *fw_file_name,
FWCfgReadCallback fw_callback, void *callback_opaque, FWCfgReadCallback fw_callback, void *callback_opaque,
AddressSpace *as) AddressSpace *as, bool read_only)
{ {
MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine()); MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
Rom *rom; Rom *rom;
@ -998,10 +998,14 @@ MemoryRegion *rom_add_blob(const char *name, const void *blob, size_t len,
char devpath[100]; char devpath[100];
void *data; void *data;
snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name); if (read_only) {
snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name);
} else {
snprintf(devpath, sizeof(devpath), "/ram@%s", fw_file_name);
}
if (mc->rom_file_has_mr) { if (mc->rom_file_has_mr) {
data = rom_set_mr(rom, OBJECT(fw_cfg), devpath); data = rom_set_mr(rom, OBJECT(fw_cfg), devpath, read_only);
mr = rom->mr; mr = rom->mr;
} else { } else {
data = rom->data; data = rom->data;
@ -1009,7 +1013,7 @@ MemoryRegion *rom_add_blob(const char *name, const void *blob, size_t len,
fw_cfg_add_file_callback(fw_cfg, fw_file_name, fw_cfg_add_file_callback(fw_cfg, fw_file_name,
fw_callback, callback_opaque, fw_callback, callback_opaque,
data, rom->datasize); data, rom->datasize, read_only);
} }
return mr; return mr;
} }

4
hw/i386/acpi-build.c
View File

@ -2806,7 +2806,7 @@ static MemoryRegion *acpi_add_rom_blob(AcpiBuildState *build_state,
uint64_t max_size) uint64_t max_size)
{ {
return rom_add_blob(name, blob->data, acpi_data_len(blob), max_size, -1, return rom_add_blob(name, blob->data, acpi_data_len(blob), max_size, -1,
name, acpi_build_update, build_state, NULL); name, acpi_build_update, build_state, NULL, true);
} }
static const VMStateDescription vmstate_acpi_build = { static const VMStateDescription vmstate_acpi_build = {
@ -2872,7 +2872,7 @@ void acpi_setup(void)
build_state->rsdp = g_memdup(tables.rsdp->data, rsdp_size); build_state->rsdp = g_memdup(tables.rsdp->data, rsdp_size);
fw_cfg_add_file_callback(pcms->fw_cfg, ACPI_BUILD_RSDP_FILE, fw_cfg_add_file_callback(pcms->fw_cfg, ACPI_BUILD_RSDP_FILE,
acpi_build_update, build_state, acpi_build_update, build_state,
build_state->rsdp, rsdp_size); build_state->rsdp, rsdp_size, true);
build_state->rsdp_mr = NULL; build_state->rsdp_mr = NULL;
} else { } else {
build_state->rsdp = NULL; build_state->rsdp = NULL;

2
hw/lm32/lm32_hwsetup.h
View File

@ -75,7 +75,7 @@ static inline void hwsetup_create_rom(HWSetup *hw,
hwaddr base) hwaddr base)
{ {
rom_add_blob("hwsetup", hw->data, TARGET_PAGE_SIZE, rom_add_blob("hwsetup", hw->data, TARGET_PAGE_SIZE,
TARGET_PAGE_SIZE, base, NULL, NULL, NULL, NULL); TARGET_PAGE_SIZE, base, NULL, NULL, NULL, NULL, true);
} }
static inline void hwsetup_add_u8(HWSetup *hw, uint8_t u) static inline void hwsetup_add_u8(HWSetup *hw, uint8_t u)

37
hw/nvram/fw_cfg.c
View File

@ -54,11 +54,13 @@
#define FW_CFG_DMA_CTL_READ 0x02 #define FW_CFG_DMA_CTL_READ 0x02
#define FW_CFG_DMA_CTL_SKIP 0x04 #define FW_CFG_DMA_CTL_SKIP 0x04
#define FW_CFG_DMA_CTL_SELECT 0x08 #define FW_CFG_DMA_CTL_SELECT 0x08
#define FW_CFG_DMA_CTL_WRITE 0x10
#define FW_CFG_DMA_SIGNATURE 0x51454d5520434647ULL /* "QEMU CFG" */ #define FW_CFG_DMA_SIGNATURE 0x51454d5520434647ULL /* "QEMU CFG" */
typedef struct FWCfgEntry { typedef struct FWCfgEntry {
uint32_t len; uint32_t len;
bool allow_write;
uint8_t *data; uint8_t *data;
void *callback_opaque; void *callback_opaque;
FWCfgReadCallback read_callback; FWCfgReadCallback read_callback;
@ -326,7 +328,7 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
FWCfgDmaAccess dma; FWCfgDmaAccess dma;
int arch; int arch;
FWCfgEntry *e; FWCfgEntry *e;
int read; int read = 0, write = 0;
dma_addr_t dma_addr; dma_addr_t dma_addr;
/* Reset the address before the next access */ /* Reset the address before the next access */
@ -353,8 +355,13 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
if (dma.control & FW_CFG_DMA_CTL_READ) { if (dma.control & FW_CFG_DMA_CTL_READ) {
read = 1; read = 1;
write = 0;
} else if (dma.control & FW_CFG_DMA_CTL_WRITE) {
read = 0;
write = 1;
} else if (dma.control & FW_CFG_DMA_CTL_SKIP) { } else if (dma.control & FW_CFG_DMA_CTL_SKIP) {
read = 0; read = 0;
write = 0;
} else { } else {
dma.length = 0; dma.length = 0;
} }
@ -374,7 +381,9 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
dma.control |= FW_CFG_DMA_CTL_ERROR; dma.control |= FW_CFG_DMA_CTL_ERROR;
} }
} }
if (write) {
dma.control |= FW_CFG_DMA_CTL_ERROR;
}
} else { } else {
if (dma.length <= (e->len - s->cur_offset)) { if (dma.length <= (e->len - s->cur_offset)) {
len = dma.length; len = dma.length;
@ -391,6 +400,14 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
dma.control |= FW_CFG_DMA_CTL_ERROR; dma.control |= FW_CFG_DMA_CTL_ERROR;
} }
} }
if (write) {
if (!e->allow_write ||
len != dma.length ||
dma_memory_read(s->dma_as, dma.address,
&e->data[s->cur_offset], len)) {
dma.control |= FW_CFG_DMA_CTL_ERROR;
}
}
s->cur_offset += len; s->cur_offset += len;
} }
@ -586,7 +603,8 @@ static const VMStateDescription vmstate_fw_cfg = {
static void fw_cfg_add_bytes_read_callback(FWCfgState *s, uint16_t key, static void fw_cfg_add_bytes_read_callback(FWCfgState *s, uint16_t key,
FWCfgReadCallback callback, FWCfgReadCallback callback,
void *callback_opaque, void *callback_opaque,
void *data, size_t len) void *data, size_t len,
bool read_only)
{ {
int arch = !!(key & FW_CFG_ARCH_LOCAL); int arch = !!(key & FW_CFG_ARCH_LOCAL);
@ -599,6 +617,7 @@ static void fw_cfg_add_bytes_read_callback(FWCfgState *s, uint16_t key,
s->entries[arch][key].len = (uint32_t)len; s->entries[arch][key].len = (uint32_t)len;
s->entries[arch][key].read_callback = callback; s->entries[arch][key].read_callback = callback;
s->entries[arch][key].callback_opaque = callback_opaque; s->entries[arch][key].callback_opaque = callback_opaque;
s->entries[arch][key].allow_write = !read_only;
} }
static void *fw_cfg_modify_bytes_read(FWCfgState *s, uint16_t key, static void *fw_cfg_modify_bytes_read(FWCfgState *s, uint16_t key,
@ -616,13 +635,14 @@ static void *fw_cfg_modify_bytes_read(FWCfgState *s, uint16_t key,
s->entries[arch][key].data = data; s->entries[arch][key].data = data;
s->entries[arch][key].len = len; s->entries[arch][key].len = len;
s->entries[arch][key].callback_opaque = NULL; s->entries[arch][key].callback_opaque = NULL;
s->entries[arch][key].allow_write = false;
return ptr; return ptr;
} }
void fw_cfg_add_bytes(FWCfgState *s, uint16_t key, void *data, size_t len) void fw_cfg_add_bytes(FWCfgState *s, uint16_t key, void *data, size_t len)
{ {
fw_cfg_add_bytes_read_callback(s, key, NULL, NULL, data, len); fw_cfg_add_bytes_read_callback(s, key, NULL, NULL, data, len, true);
} }
void fw_cfg_add_string(FWCfgState *s, uint16_t key, const char *value) void fw_cfg_add_string(FWCfgState *s, uint16_t key, const char *value)
@ -749,7 +769,7 @@ static int get_fw_cfg_order(FWCfgState *s, const char *name)
void fw_cfg_add_file_callback(FWCfgState *s, const char *filename, void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
FWCfgReadCallback callback, void *callback_opaque, FWCfgReadCallback callback, void *callback_opaque,
void *data, size_t len) void *data, size_t len, bool read_only)
{ {
int i, index, count; int i, index, count;
size_t dsize; size_t dsize;
@ -811,7 +831,8 @@ void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
} }
fw_cfg_add_bytes_read_callback(s, FW_CFG_FILE_FIRST + index, fw_cfg_add_bytes_read_callback(s, FW_CFG_FILE_FIRST + index,
callback, callback_opaque, data, len); callback, callback_opaque, data, len,
read_only);
s->files->f[index].size = cpu_to_be32(len); s->files->f[index].size = cpu_to_be32(len);
s->files->f[index].select = cpu_to_be16(FW_CFG_FILE_FIRST + index); s->files->f[index].select = cpu_to_be16(FW_CFG_FILE_FIRST + index);
@ -824,7 +845,7 @@ void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
void fw_cfg_add_file(FWCfgState *s, const char *filename, void fw_cfg_add_file(FWCfgState *s, const char *filename,
void *data, size_t len) void *data, size_t len)
{ {
fw_cfg_add_file_callback(s, filename, NULL, NULL, data, len); fw_cfg_add_file_callback(s, filename, NULL, NULL, data, len, true);
} }
void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *fw_cfg_modify_file(FWCfgState *s, const char *filename,
@ -847,7 +868,7 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename,
} }
} }
/* add new one */ /* add new one */
fw_cfg_add_file_callback(s, filename, NULL, NULL, data, len); fw_cfg_add_file_callback(s, filename, NULL, NULL, data, len, true);
return NULL; return NULL;
} }

7
include/hw/loader.h
View File

@ -180,7 +180,8 @@ MemoryRegion *rom_add_blob(const char *name, const void *blob, size_t len,
size_t max_len, hwaddr addr, size_t max_len, hwaddr addr,
const char *fw_file_name, const char *fw_file_name,
FWCfgReadCallback fw_callback, FWCfgReadCallback fw_callback,
void *callback_opaque, AddressSpace *as); void *callback_opaque, AddressSpace *as,
bool read_only);
int rom_add_elf_program(const char *name, void *data, size_t datasize, int rom_add_elf_program(const char *name, void *data, size_t datasize,
size_t romsize, hwaddr addr, AddressSpace *as); size_t romsize, hwaddr addr, AddressSpace *as);
int rom_check_and_register_reset(void); int rom_check_and_register_reset(void);
@ -194,7 +195,7 @@ void hmp_info_roms(Monitor *mon, const QDict *qdict);
#define rom_add_file_fixed(_f, _a, _i) \ #define rom_add_file_fixed(_f, _a, _i) \
rom_add_file(_f, NULL, _a, _i, false, NULL, NULL) rom_add_file(_f, NULL, _a, _i, false, NULL, NULL)
#define rom_add_blob_fixed(_f, _b, _l, _a) \ #define rom_add_blob_fixed(_f, _b, _l, _a) \
rom_add_blob(_f, _b, _l, _l, _a, NULL, NULL, NULL, NULL) rom_add_blob(_f, _b, _l, _l, _a, NULL, NULL, NULL, NULL, true)
#define rom_add_file_mr(_f, _mr, _i) \ #define rom_add_file_mr(_f, _mr, _i) \
rom_add_file(_f, NULL, 0, _i, false, _mr, NULL) rom_add_file(_f, NULL, 0, _i, false, _mr, NULL)
#define rom_add_file_as(_f, _as, _i) \ #define rom_add_file_as(_f, _as, _i) \
@ -202,7 +203,7 @@ void hmp_info_roms(Monitor *mon, const QDict *qdict);
#define rom_add_file_fixed_as(_f, _a, _i, _as) \ #define rom_add_file_fixed_as(_f, _a, _i, _as) \
rom_add_file(_f, NULL, _a, _i, false, NULL, _as) rom_add_file(_f, NULL, _a, _i, false, NULL, _as)
#define rom_add_blob_fixed_as(_f, _b, _l, _a, _as) \ #define rom_add_blob_fixed_as(_f, _b, _l, _a, _as) \
rom_add_blob(_f, _b, _l, _l, _a, NULL, NULL, NULL, _as) rom_add_blob(_f, _b, _l, _l, _a, NULL, NULL, NULL, _as, true)
#define PC_ROM_MIN_VGA 0xc0000 #define PC_ROM_MIN_VGA 0xc0000
#define PC_ROM_MIN_OPTION 0xc8000 #define PC_ROM_MIN_OPTION 0xc8000

3
include/hw/nvram/fw_cfg.h
View File

@ -136,6 +136,7 @@ void fw_cfg_add_file(FWCfgState *s, const char *filename, void *data,
* @callback_opaque: argument to be passed into callback function * @callback_opaque: argument to be passed into callback function
* @data: pointer to start of item data * @data: pointer to start of item data
* @len: size of item data * @len: size of item data
* @read_only: is file read only
* *
* Add a new NAMED fw_cfg item as a raw "blob" of the given size. The data * Add a new NAMED fw_cfg item as a raw "blob" of the given size. The data
* referenced by the starting pointer is only linked, NOT copied, into the * referenced by the starting pointer is only linked, NOT copied, into the
@ -151,7 +152,7 @@ void fw_cfg_add_file(FWCfgState *s, const char *filename, void *data,
*/ */
void fw_cfg_add_file_callback(FWCfgState *s, const char *filename, void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
FWCfgReadCallback callback, void *callback_opaque, FWCfgReadCallback callback, void *callback_opaque,
void *data, size_t len); void *data, size_t len, bool read_only);
/** /**
* fw_cfg_modify_file: * fw_cfg_modify_file: