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Shuran Liu d0e177e176 bpf: Fix verifier assumptions of bpf_d_path's output buffer 
[ Upstream commit ac44dcc788 ]

Commit 37cce22dbd ("bpf: verifier: Refactor helper access type
tracking") started distinguishing read vs write accesses performed by
helpers.

The second argument of bpf_d_path() is a pointer to a buffer that the
helper fills with the resulting path. However, its prototype currently
uses ARG_PTR_TO_MEM without MEM_WRITE.

Before 37cce22dbd, helper accesses were conservatively treated as
potential writes, so this mismatch did not cause issues. Since that
commit, the verifier may incorrectly assume that the buffer contents
are unchanged across the helper call and base its optimizations on this
wrong assumption. This can lead to misbehaviour in BPF programs that
read back the buffer, such as prefix comparisons on the returned path.

Fix this by marking the second argument of bpf_d_path() as
ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the
write to the caller-provided buffer.

Fixes: 37cce22dbd ("bpf: verifier: Refactor helper access type tracking")
Co-developed-by: Zesen Liu <ftyg@live.com>
Signed-off-by: Zesen Liu <ftyg@live.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20251206141210.3148-2-electronlsr@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-01-02 12:56:37 +01:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.