[PATCH V2 3/3] efi: pstore: Add module parameter for setting the record size

Guilherme G. Piccoli
2022-10-13T21:11:45Z

By default, the efi-pstore backend hardcode the UEFI variable size
as 1024 bytes. The historical reasons for that were discussed by
Ard in threads [0][1]:

"there is some cargo cult from prehistoric EFI times going
on here, it seems. Or maybe just misinterpretation of the maximum
size for the variable *name* vs the variable itself.".

"OVMF has
OvmfPkg/OvmfPkgX64.dsc:
gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x2000
OvmfPkg/OvmfPkgX64.dsc:
gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x8400

where the first one is without secure boot and the second with secure
boot. Interestingly, the default is

gEfiMdeModulePkgTokenSpaceGuid.PcdMaxVariableSize|0x400

so this is probably where this 1k number comes from."

With that, and since there is not such a limit in the UEFI spec, we
have the confidence to hereby add a module parameter to enable advanced
users to change the UEFI record size for efi-pstore data collection,
this way allowing a much easier reading of the collected log, which is
not scattered anymore among many small files.

Through empirical analysis we observed that extreme low values (like 8
bytes) could eventually cause writing issues, so given that and the OVMF
default discussed, we limited the minimum value to 1024 bytes, which also
is still the default.

[0] https://lore.kernel.org/lkml/CAMj1kXF4UyRMh2Y_KakeNBHvkHhTtavASTAxXinDO1rhPe_wYg@mail.gmail.com/
[1] https://lore.kernel.org/lkml/CAMj1kXFy-2KddGu+dgebAdU9v2sindxVoiHLWuVhqYw+R=kqng@mail.gmail.com/

Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---

V2:
- Fixed a memory corruption bug in the code (that wasn't causing
trouble before due to the fixed sized of record_size), thanks
Ard for spotting this!

- Added Ard's archeology in the commit message plus a comment
with the reasoning behind the minimum value.

drivers/firmware/efi/efi-pstore.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
index 97a9e84840a0..827e32427ddb 100644
--- a/drivers/firmware/efi/efi-pstore.c
+++ b/drivers/firmware/efi/efi-pstore.c
@@ -10,7 +10,9 @@ MODULE_IMPORT_NS(EFIVAR);

#define DUMP_NAME_LEN 66

-#define EFIVARS_DATA_SIZE_MAX 1024
+static unsigned int record_size = 1024;
+module_param(record_size, uint, 0444);
+MODULE_PARM_DESC(record_size, "size of each pstore UEFI var (in bytes, min/default=1024)");

static bool efivars_pstore_disable =
IS_ENABLED(CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE);
@@ -30,7 +32,7 @@ static int efi_pstore_open(struct pstore_info *psi)
if (err)
return err;

- psi->data = kzalloc(EFIVARS_DATA_SIZE_MAX, GFP_KERNEL);
+ psi->data = kzalloc(record_size, GFP_KERNEL);
if (!psi->data)
return -ENOMEM;

@@ -52,7 +54,7 @@ static inline u64 generic_id(u64 timestamp, unsigned int part, int count)
static int efi_pstore_read_func(struct pstore_record *record,
efi_char16_t *varname)
{
- unsigned long wlen, size = EFIVARS_DATA_SIZE_MAX;
+ unsigned long wlen, size = record_size;
char name[DUMP_NAME_LEN], data_type;
efi_status_t status;
int cnt;
@@ -133,7 +135,7 @@ static ssize_t efi_pstore_read(struct pstore_record *record)
efi_status_t status;

for (;;) {
- varname_size = EFIVARS_DATA_SIZE_MAX;
+ varname_size = record_size;

/*
* If this is the first read() call in the pstore enumeration,
@@ -224,11 +226,20 @@ static __init int efivars_pstore_init(void)
if (efivars_pstore_disable)
return 0;

- efi_pstore_info.buf = kmalloc(4096, GFP_KERNEL);
+ /*
+ * Notice that 1024 is the minimum here to prevent issues with
+ * decompression algorithms that were spotted during tests;
+ * even in the case of not using compression, smaller values would
+ * just pollute more the pstore FS with many small collected files.
+ */
+ if (record_size < 1024)
+ record_size = 1024;
+
+ efi_pstore_info.buf = kmalloc(record_size, GFP_KERNEL);
if (!efi_pstore_info.buf)
return -ENOMEM;

- efi_pstore_info.bufsize = 1024;
+ efi_pstore_info.bufsize = record_size;

if (pstore_register(&efi_pstore_info)) {
kfree(efi_pstore_info.buf);
--
2.38.0