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fix: write NUL terminator for MAM coherency signature #601

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fix: write NUL terminator for MAM coherency signature #601

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68b9660
fix: write NUL terminator for MAM coherency signature
hugo-hur Jun 9, 2026
1e605f4
refactor: harden MAM coherency signature handling
hugo-hur Jun 14, 2026
2d1490a
refactor: guard sibling MAM page buffers with _Static_assert
hugo-hur Jun 14, 2026
cc042cf
Merge branch 'LinearTapeFileSystem:main' into fix-mam-coherency-signa…
hugo-hur Aug 6, 2026
37489c6
refactor: remove redundant MAM buffer static assertions
hugo-hur Aug 6, 2026
10ef719
refactor: name the MAM coherency page field offsets
hugo-hur Aug 6, 2026
da49d74
refactor: cleaning up ai slop comments
hugo-hur Aug 8, 2026
c64a167
refactor: remove unnecessary memset
hugo-hur Aug 8, 2026
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59 changes: 30 additions & 29 deletions src/libltfs/tape.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1674,7 +1674,7 @@ int tape_get_volume_change_reference(struct device_data *dev, uint64_t *volume_c
sizeof(vcr_data));

if (ret == 0) {
*volume_change_ref = (uint64_t)ltfs_betou32(vcr_data + 5);
*volume_change_ref = (uint64_t)ltfs_betou32(vcr_data + TC_MAM_PAGE_HEADER_SIZE);
if (*volume_change_ref == UINT32_MAX)
*volume_change_ref = UINT64_MAX; /* maintain "unusable VCR" state correctly */
} else {
Expand Down Expand Up @@ -1705,9 +1705,9 @@ int tape_get_cart_coherency(struct device_data *dev, const tape_partition_t part
sizeof(coh_data));

if (ret == 0) {
uint16_t id = ltfs_betou16(coh_data);
uint16_t len = ltfs_betou16(coh_data + 3);
uint8_t vcr_size = coh_data[5];
uint16_t id = ltfs_betou16(coh_data + TC_MAM_ATTR_ID_OFFSET);
uint16_t len = ltfs_betou16(coh_data + TC_MAM_ATTR_LENGTH_OFFSET);
uint8_t vcr_size = coh_data[TC_MAM_COH_VCR_SIZE_OFFSET];

if (id != TC_MAM_PAGE_COHERENCY) {
ltfsmsg(LTFS_WARN, 12058W, id);
Expand All @@ -1723,35 +1723,37 @@ int tape_get_cart_coherency(struct device_data *dev, const tape_partition_t part
coh->set_id = 0;

switch (vcr_size) {
case 8:
coh->volume_change_ref = ltfs_betou64(coh_data + 6);
case TC_MAM_COH_VCR_LENGTH:
coh->volume_change_ref = ltfs_betou64(coh_data + TC_MAM_COH_VCR_OFFSET);
break;
default:
ltfsmsg(LTFS_WARN, 12060W, vcr_size);
return -LTFS_UNEXPECTED_VALUE;
}

coh->count = ltfs_betou64(coh_data + 14);
coh->set_id = ltfs_betou64(coh_data + 22);
coh->count = ltfs_betou64(coh_data + TC_MAM_COH_COUNT_OFFSET);
coh->set_id = ltfs_betou64(coh_data + TC_MAM_COH_SETID_OFFSET);

/* Allow ap_clent_specific_len is 42 and 43 to keep backward compatibility.
* It should be 43 but in LTFS 1.0 and 1.0.1, it was set 42 as a code bug...
*/
uint16_t ap_clent_specific_len = ltfs_betou16(coh_data + 30);
if (ap_clent_specific_len != 42 && ap_clent_specific_len != 43) {
uint16_t ap_clent_specific_len = ltfs_betou16(coh_data + TC_MAM_COH_APPINFO_LEN_OFFSET);
if (ap_clent_specific_len != TC_MAM_COH_APPINFO_LENGTH_LEGACY &&
ap_clent_specific_len != TC_MAM_COH_APPINFO_LENGTH) {
ltfsmsg(LTFS_WARN, 12061W, ap_clent_specific_len);
return -LTFS_UNEXPECTED_VALUE;
} else if (strncmp((char *)coh_data + 32, "LTFS", sizeof("LTFS")) != 0) {
} else if (strncmp((char *)coh_data + TC_MAM_COH_SIGNATURE_OFFSET, TC_MAM_COHERENCY_SIGNATURE,
sizeof(TC_MAM_COHERENCY_SIGNATURE)) != 0) {
ltfsmsg(LTFS_WARN, 12062W);
return -LTFS_UNEXPECTED_VALUE;
}

memcpy(coh->uuid, coh_data + 37, 37);
memcpy(coh->uuid, coh_data + TC_MAM_COH_UUID_OFFSET, sizeof(coh->uuid));

/* Don't need to check the version field because the values parsed above are guaranteed
* to be supported in every version of the LTFS MAM parameters.
*/
coh->version = coh_data[74];
coh->version = coh_data[TC_MAM_COH_VERSION_OFFSET];
} else
ltfsmsg(LTFS_WARN, 12057W, ret);

Expand All @@ -1772,25 +1774,24 @@ int tape_set_cart_coherency(struct device_data *dev, const tape_partition_t part
CHECK_ARG_NULL(dev, -LTFS_NULL_ARG);
CHECK_ARG_NULL(dev->backend, -LTFS_NULL_ARG);

ltfs_u16tobe(coh_data, TC_MAM_PAGE_COHERENCY);
coh_data[2] = 0;
ltfs_u16tobe(coh_data + 3, TC_MAM_PAGE_COHERENCY_SIZE);
coh_data[5] = 0x08; /* Size of Volume Change Reference Value (VCR)*/
ltfs_u64tobe(coh_data + 6, coh->volume_change_ref);
ltfs_u64tobe(coh_data + 14, coh->count); /* VOLUME COHERENCY COUNT */
ltfs_u64tobe(coh_data + 22, coh->set_id); /* VOLUME COHERENCY SET IDENTIFIER */
/* APPLICATION CLIENT SPECIFIC INFORMATION LENGTH */
coh_data[30] = 0; /* Size of APPLICATION CLIENT SPECIFIC INFORMATION (Byte 1) */
coh_data[31] = 43; /* Size of APPLICATION CLIENT SPECIFIC INFORMATION (Byte 0) */
/* Size of the buffer to insert 'LTFS' needs to be size of 5 for the 4 letters and the null terminator*/
arch_strncpy((char *)coh_data + 32,"LTFS", 5, 4);
memcpy(coh_data + 37, coh->uuid, 37);
ltfs_u16tobe(coh_data + TC_MAM_ATTR_ID_OFFSET, TC_MAM_PAGE_COHERENCY);
coh_data[TC_MAM_ATTR_FORMAT_OFFSET] = 0;
ltfs_u16tobe(coh_data + TC_MAM_ATTR_LENGTH_OFFSET, TC_MAM_PAGE_COHERENCY_SIZE);
coh_data[TC_MAM_COH_VCR_SIZE_OFFSET] = TC_MAM_COH_VCR_LENGTH; /* Size of Volume Change Reference (VCR) */
ltfs_u64tobe(coh_data + TC_MAM_COH_VCR_OFFSET, coh->volume_change_ref);
ltfs_u64tobe(coh_data + TC_MAM_COH_COUNT_OFFSET, coh->count); /* VOLUME COHERENCY COUNT */
ltfs_u64tobe(coh_data + TC_MAM_COH_SETID_OFFSET, coh->set_id); /* VOLUME COHERENCY SET IDENTIFIER */
/* APPLICATION CLIENT SPECIFIC INFORMATION LENGTH: signature + UUID + version */
ltfs_u16tobe(coh_data + TC_MAM_COH_APPINFO_LEN_OFFSET, TC_MAM_COH_APPINFO_LENGTH);
/* memcpy instead of arch_strncpy to deterministically copy all bytes of the coherency signature including NULL terminator */
memcpy(coh_data + TC_MAM_COH_SIGNATURE_OFFSET, TC_MAM_COHERENCY_SIGNATURE, sizeof(TC_MAM_COHERENCY_SIGNATURE));
memcpy(coh_data + TC_MAM_COH_UUID_OFFSET, coh->uuid, sizeof(coh->uuid));
/*
Version field
0: GA and PGA1
1: From PGA2
*/
coh_data[74] = coh->version; /* version field should be specified before calling this function */
coh_data[TC_MAM_COH_VERSION_OFFSET] = coh->version; /* version field should be specified before calling this function */

ret = dev->backend->write_attribute(dev->backend_data, part, coh_data, sizeof(coh_data));
if (ret < 0)
Expand All @@ -1813,8 +1814,8 @@ int tape_get_cart_volume_lock_status(struct device_data *dev, int *status)
sizeof(attr_data));

if (ret == 0) {
uint16_t id = ltfs_betou16(attr_data);
uint16_t len = ltfs_betou16(attr_data + 3);
uint16_t id = ltfs_betou16(attr_data + TC_MAM_ATTR_ID_OFFSET);
uint16_t len = ltfs_betou16(attr_data + TC_MAM_ATTR_LENGTH_OFFSET);

if (id != TC_MAM_LOCKED_MAM) {
ltfsmsg(LTFS_WARN, 17196W, id);
Expand Down
21 changes: 21 additions & 0 deletions src/libltfs/tape_ops.h
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Original file line number Diff line number Diff line change
Expand Up @@ -250,6 +250,27 @@ typedef enum {
#define TC_MAM_PAGE_VCR_SIZE (0x4) /* Size of Volume Change Reference */
#define TC_MAM_PAGE_COHERENCY (0x080C)
#define TC_MAM_PAGE_COHERENCY_SIZE (0x46)
#define TC_MAM_COHERENCY_SIGNATURE "LTFS" /* Volume coherency signature; 4 chars + NUL, reader compares all 5 */

/* Byte offsets of the common MAM attribute header fields. The attribute value
* begins at TC_MAM_PAGE_HEADER_SIZE. */
#define TC_MAM_ATTR_ID_OFFSET (0) /* 2-byte attribute identifier */
#define TC_MAM_ATTR_FORMAT_OFFSET (2) /* 1-byte format byte */
#define TC_MAM_ATTR_LENGTH_OFFSET (3) /* 2-byte attribute length */

/* Byte offsets of the fields within the volume coherency attribute
* (TC_MAM_PAGE_COHERENCY), measured from the start of the attribute. */
#define TC_MAM_COH_VCR_SIZE_OFFSET (5) /* 1-byte length of the VCR field */
#define TC_MAM_COH_VCR_OFFSET (6) /* 8-byte volume change reference */
#define TC_MAM_COH_COUNT_OFFSET (14) /* 8-byte coherency count */
#define TC_MAM_COH_SETID_OFFSET (22) /* 8-byte coherency set identifier */
#define TC_MAM_COH_APPINFO_LEN_OFFSET (30) /* 2-byte app client specific info length */
#define TC_MAM_COH_SIGNATURE_OFFSET (32) /* 5-byte "LTFS\0" signature */
#define TC_MAM_COH_UUID_OFFSET (37) /* volume UUID (sizeof tc_coherency.uuid) */
#define TC_MAM_COH_VERSION_OFFSET (74) /* 1-byte format version */
#define TC_MAM_COH_VCR_LENGTH (8) /* value stored at the VCR size field */
#define TC_MAM_COH_APPINFO_LENGTH (43) /* signature + UUID + version byte count */
#define TC_MAM_COH_APPINFO_LENGTH_LEGACY (42) /* LTFS 1.0/1.0.1 wrote 42 (off-by-one) */

#define TC_MAM_APP_VENDER (0x0800)
#define TC_MAM_APP_VENDER_SIZE (0x8)
Expand Down
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