[U-Boot] [PATCH] VFAT: fix processing of scattered long file name entries

[Wolfgang Denk]

Dear Mikhail Zolotaryov,

In message <1283954763-22436-1-git-send-email-lebon at lebon.org.ua> you wrote:
> The U-Boot code has the following bugs related to the processing of Long File
> Name (LFN) entries scattered across several clusters/sectors :
> 
> 1) get_vfatname() function is designed to gather scattered LFN entries by
> cluster chain processing - that doesn't work for FAT12/16 root directory.
> In other words, the function expects the following input data:
>  1.1) FAT32 directory (which is cluster chain based);
>         OR
>  1.2) FAT12/16 non-root directory (which is also cluster chain based);
>         OR
>  1.3) FAT12/16 root directory (allocated as contiguous sectors area), but
>  all necessary information MUST be within the input buffer of filesystem cluster
>  size (thus cluster-chain jump is never initiated).
> 
> In order to accomplish the last condition, root directory parsing code in
> do_fat_read() uses the following trick: read-out cluster-size block, process
> only first sector (512 bytes), then shift 512 forward, read-out cluster-size
> block and so on. This works great unless cluster size is equal to 512 bytes
> (in a case you have a small partition), or long file name entries are scattered
> across three sectors, see 4) for details.
> 
> 2) Despite of the fact that get_vfatname() supports FAT32 root directory
> browsing, do_fat_read() function doesn't send current cluster number correctly,
> so root directory look-up doesn't work correctly.
> 
> 3) get_vfatname() doesn't gather scattered entries correctly also is the case
> when all LFN entries are located at the end of the source cluster, but real
> directory entry (which must be returned) is at the only beginning of the
> next one. No error detected, the resulting directory entry returned contains
> a semi-random information (wrong size, wrong start cluster number and so on)
> i.e. the entry is not accessible.
> 
> 4) LFN (VFAT) allows up to 20 entries (slots) each containing 26 bytes (13
> UTF-16 code units) to represent a single long file name i.e. up to 520 bytes.
> U-Boot allocates 256 bytes buffer instead, i.e. 10 or more LFN slots record
> may cause buffer overflow / memory corruption.
> Also, it's worth to mention that 20+1 slots occupy 672 bytes space which may
> take more than one cluster of 512 bytes (medium-size FAT32 or small FAT16
> partition) - get_vfatname() function doesn't support such case as well.
> 
> The patch attached fixes these problems in the following way:
> - keep using 256 bytes buffer for a long file name, but safely prevent a
> possible buffer overflow (skip LFN processing, if it contains 10 or more
> slots).
> 
> - explicitly specify FAT12/16 root directory parsing buffer size, instead
> of relying on cluster size. The value used is a double sector size (to store
> current sector and the next one). This fixes the first problem and increases
> performance on big FAT12/16 partitions;
> 
> - send current cluster number (FAT32) to get_vfatname() during root
> directory processing;
> 
> - use LFN counter to seek the real directory entry in get_vfatname() - fixes the
> third problem;
> 
> - skip deleted entries in the root directory (to prevent bogus buffer
> overflow detection and LFN counter steps).
> 
> Note: it's not advised to split up the patch, because a separate part may
> operate incorrectly.
> 
> Signed-off-by: Mikhail Zolotaryov <lebon at lebon.org.ua>
> ---
>  fs/fat/fat.c  |   49 +++++++++++++++++++++++++++++++++++++------------
>  include/fat.h |    4 ++++
>  2 files changed, 41 insertions(+), 12 deletions(-)

Applied, thanks.

Best regards,

Wolfgang Denk

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