Re: [PATCH v4 04/11] seq_buf: Clear what a writer did not claim when a seq_buf overflows

seq_buf: Clear what a writer did not claim when a seq_buf overflows

TL;DR: Commit message only: the claim that seq_buf_path() was "never exported,
unlike the other writers" looks inaccurate, since several other writers are
also unexported. The code and export look correct.

    seq_buf: Clear what a writer did not claim when a seq_buf overflows

    Using seq_buf_set_overflow() would leave the bytes between "len"
    and "size" untouched, so if seq_buf_str() is used on an overflowed
    seq_buf, those bytes may be exposed. For any paths that don't claim
    partially written bytes, by setting "len = size" before calling
    seq_buf_set_overflow(), wipe the unclaimed bytes. The seq_buf_puts()
    and related APIs already claim those bytes now, so only the unclaimed
    cases remain. A specific example of this was seq_buf_path() which uses
    d_path() and would write to the tail before discovering it was out
    of space, and would correctly mark a seq_buf as overflowed, but the
    path fragment would be left over.

    Clear from len to the end of the buffer in seq_buf_set_overflow(), which
    every overflow goes through, including seq_buf_commit() with a negative
    count.

    Add a test that fills a seq_buf, leaves it too little room for a path, and
    checks that nothing of the path is left in the buffer. The tests run before
    anything writable is mounted, so it takes its file from shmem.

    seq_buf_path() was never exported, unlike the other writers, so the
    test failed to link as a module. Export it.

    Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
    and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0, and the
    test builds as a module (CONFIG_SEQ_BUF_KUNIT_TEST=m).

    Assisted-by: LLM
    Reviewed-by: Andy Shevchenko [off-list ref]
    Signed-off-by: Kees Cook [off-list ref]

This isn't a bug, but is the comparison in "seq_buf_path() was never
exported, unlike the other writers" accurate?

In lib/seq_buf.c the only writers with an export are seq_buf_printf(),
seq_buf_puts(), seq_buf_putc(), seq_buf_putmem() and seq_buf_putmem_hex()
(plus seq_buf_do_printk(), which is not a writer).  seq_buf_vprintf(),
seq_buf_bprintf() and seq_buf_hex_dump() also write into a seq_buf and have
no EXPORT_SYMBOL_GPL().

The reason for the export, that the new KUnit test calls seq_buf_path() and
has to link with CONFIG_SEQ_BUF_KUNIT_TEST=m, looks correct.  Could the
sentence drop the "unlike the other writers" part, for example:

  seq_buf_path() was not exported, so the test failed to link as a module.
  Export it.


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