On Thu, 13 Oct 2022 at 23:11, Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> wrote: > > 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@xxxxxxxxxxxxxx/ > [1] https://lore.kernel.org/lkml/CAMj1kXFy-2KddGu+dgebAdU9v2sindxVoiHLWuVhqYw+R=kqng@xxxxxxxxxxxxxx/ > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> > --- > > > 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; > I don't think we need this - this is the size of the variable name not the variable itself. > /* > * 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 >
