Every SetVariable() call triggers efi_var_mem_ins() followed by efi_var_to_storage(), even when the variable value is not actually changing. This is unfriendly to flash-backed stores that suffer wear from unnecessary erase/write cycles.
Add a change-detection path to efi_var_mem_ins(): when size2 == 0 (i.e. not an append) and the caller passes a non-NULL changep flag, look up the existing variable and compare attributes, length, time and data byte-by-byte. If everything matches, set *changep = false and return EFI_SUCCESS without touching the variable buffer. Both efi_set_variable_int() and efi_set_variable_runtime() now check the flag and skip efi_var_mem_del() / efi_var_to_storage() when nothing changed. Signed-off-by: Michal Simek <[email protected]> --- include/efi_variable.h | 8 +++++++- lib/efi_loader/efi_var_common.c | 2 +- lib/efi_loader/efi_var_mem.c | 29 ++++++++++++++++++++++++++++- lib/efi_loader/efi_variable.c | 27 ++++++++++++++++++--------- 4 files changed, 54 insertions(+), 12 deletions(-) diff --git a/include/efi_variable.h b/include/efi_variable.h index fc1184e5ca1a..c3229c717d89 100644 --- a/include/efi_variable.h +++ b/include/efi_variable.h @@ -216,6 +216,11 @@ void efi_var_mem_del(struct efi_var_entry *var); * The variable is appended without checking if a variable of the same name * already exists. The two data buffers are concatenated. * + * When @changep is non-NULL and @size2 is 0, the function compares the new + * value against an existing variable with the same name and vendor. If + * attributes and data are identical the insertion is skipped and *@changep + * is set to false, avoiding superfluous writes. + * * @variable_name: variable name * @vendor: GUID * @attributes: variable attributes @@ -224,13 +229,14 @@ void efi_var_mem_del(struct efi_var_entry *var); * @size2: size of the second data field * @data2: second data buffer * @time: time of authentication (as seconds since start of epoch) + * @changep: pointer to change flag (may be NULL) * Result: status code */ efi_status_t efi_var_mem_ins(const u16 *variable_name, const efi_guid_t *vendor, u32 attributes, const efi_uintn_t size1, const void *data1, const efi_uintn_t size2, const void *data2, - const u64 time); + const u64 time, bool *changep); /** * efi_var_mem_free() - determine free memory for variables diff --git a/lib/efi_loader/efi_var_common.c b/lib/efi_loader/efi_var_common.c index c89a4fce4ff8..d63c2d1b1cd8 100644 --- a/lib/efi_loader/efi_var_common.c +++ b/lib/efi_loader/efi_var_common.c @@ -526,7 +526,7 @@ efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe) continue; ret = efi_var_mem_ins(var->name, &var->guid, var->attr, var->length, data, 0, NULL, - var->time); + var->time, NULL); if (ret != EFI_SUCCESS) log_err("Failed to set EFI variable %ls\n", var->name); } diff --git a/lib/efi_loader/efi_var_mem.c b/lib/efi_loader/efi_var_mem.c index 31180df9e3a0..e364672db81a 100644 --- a/lib/efi_loader/efi_var_mem.c +++ b/lib/efi_loader/efi_var_mem.c @@ -159,12 +159,39 @@ efi_status_t __efi_runtime efi_var_mem_ins( const efi_guid_t *vendor, u32 attributes, const efi_uintn_t size1, const void *data1, const efi_uintn_t size2, const void *data2, - const u64 time) + const u64 time, bool *changep) { u16 *data; struct efi_var_entry *var; u32 var_name_len; + if (changep) + *changep = true; + + /* + * If this is not an append (size2 == 0), check whether the variable + * already exists with identical attributes and data. When nothing + * changed we can skip the write and avoid superfluous erases. + */ + if (!size2 && changep) { + struct efi_var_entry *old; + + old = efi_var_mem_find(vendor, variable_name, NULL); + if (old && old->attr == attributes && + old->length == size1 && old->time == time) { + u16 *old_data; + + for (old_data = old->name; *old_data; ++old_data) + ; + ++old_data; + + if (!memcmp(old_data, data1, size1)) { + *changep = false; + return EFI_SUCCESS; + } + } + } + var = (struct efi_var_entry *) ((uintptr_t)efi_var_buf + efi_var_buf->length); var_name_len = u16_strlen(variable_name) + 1; diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c index 8512bc20f11e..9923936c1b59 100644 --- a/lib/efi_loader/efi_variable.c +++ b/lib/efi_loader/efi_variable.c @@ -277,6 +277,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name, struct efi_var_entry *var; efi_uintn_t ret; bool append, delete; + bool changed = false; u64 time = 0; enum efi_auth_var_type var_type; @@ -366,6 +367,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name, if (delete) { /* EFI_NOT_FOUND has been handled before */ attributes = var->attr; + changed = true; ret = EFI_SUCCESS; } else if (append && var) { /* @@ -380,15 +382,19 @@ efi_status_t efi_set_variable_int(const u16 *variable_name, ret = efi_var_mem_ins(variable_name, vendor, attributes & ~EFI_VARIABLE_APPEND_WRITE, var->length, old_data, data_size, data, - time); + time, &changed); } else { ret = efi_var_mem_ins(variable_name, vendor, attributes, - data_size, data, 0, NULL, time); + data_size, data, 0, NULL, time, + &changed); } if (ret != EFI_SUCCESS) return ret; + if (!changed) + return EFI_SUCCESS; + efi_var_mem_del(var); if (var_type == EFI_AUTH_VAR_PK) @@ -396,10 +402,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name, else ret = EFI_SUCCESS; - /* - * Write non-volatile EFI variables - * TODO: check if a value change has occured to avoid superfluous writes - */ + /* Write non-volatile EFI variables to storage */ if (attributes & EFI_VARIABLE_NON_VOLATILE) { if (IS_ENABLED(CONFIG_EFI_VARIABLE_NO_STORE)) return EFI_SUCCESS; @@ -498,6 +501,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor, struct efi_var_entry *var; efi_uintn_t ret; bool append, delete; + bool changed = false; u64 time = 0; if (!IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE)) @@ -549,6 +553,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor, if (delete) { /* EFI_NOT_FOUND has been handled before */ attributes = var->attr; + changed = true; ret = EFI_SUCCESS; } else if (append && var) { u16 *old_data = (void *)((uintptr_t)var->name + @@ -556,15 +561,19 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor, ret = efi_var_mem_ins(variable_name, vendor, attributes, var->length, old_data, data_size, data, - time); + time, &changed); } else { ret = efi_var_mem_ins(variable_name, vendor, attributes, - data_size, data, 0, NULL, time); + data_size, data, 0, NULL, time, + &changed); } if (ret != EFI_SUCCESS) return ret; - /* We are always inserting new variables, get rid of the old copy */ + + if (!changed) + return EFI_SUCCESS; + efi_var_mem_del(var); return EFI_SUCCESS; -- 2.43.0 base-commit: 1e8eb1c27210e3beb5d3f4402a40d2e8886bcf9f branch: debian-sent3
