rcache contains invalid data after read error, causing incorrect data returned by lfs_read()

[simaodangan]

Description

After a file is successfully opened and written, if the SD card fails (for example, the card is removed), subsequent operations may behave incorrectly.

A simplified example:

file = lfs_open();                     // ok
lfs_write(file, "something 1");        // ok

// now remove sd card, so lfs->cfg->read() returns a negative error code
lfs_write(file, "something 2");        // failed

error_code = lfs_read(file, buffer);   // return code is "OK", but data is invalid

In this scenario:

• lfs_write("something 2") correctly returns an error
• However, lfs_read() returns success, but the returned data is invalid

This appears to be caused by rcache containing stale/invalid data.

Root Cause Analysis

lfs_write(file, "something 2") calls:

lfs_file_flushedwrite()
  → lfs_ctz_find()
      → lfs_bd_read()

Inside lfs_bd_read():

rcache->block = block;
rcache->off = lfs_aligndown(off, lfs->cfg->read_size);
rcache->size = lfs_min(
        lfs_min(
            lfs_alignup(off+hint, lfs->cfg->read_size),
            lfs->cfg->block_size)
        - rcache->off,
        lfs->cfg->cache_size);

int err = lfs->cfg->read(lfs->cfg, rcache->block,
        rcache->off, rcache->buffer, rcache->size);

LFS_ASSERT(err <= 0);

if (err) {
    return err;
}

If lfs->cfg->read() returns an error:

• rcache->block, rcache->off, and rcache->size are already updated
• rcache->buffer still contains invalid / partial data
• The cache is not cleared

Later, lfs_read() may use this invalid rcache data and return incorrect content.

Proposed Fix

Would the following change be appropriate?

if (err) {
    lfs_cache_drop(lfs, rcache);
    return err;
}

This ensures that invalid cache data is not reused after a failed read.

Question

Is this the correct fix, or is there a reason why rcache should remain unchanged on read failure?

[simaodangan]

I pushed the fix for request: #1206