Commit d7f0087381 ("bpf: try harder to deduce register bounds from
different numeric domains") added a second call to __reg_deduce_bounds
in reg_bounds_sync because a single call wasn't enough to converge to a
fixed point in terms of register bounds.
With patch "bpf: Improve bounds when s64 crosses sign boundary" from
this series, Eduard noticed that calling __reg_deduce_bounds twice isn't
enough anymore to converge. The first selftest added in "selftests/bpf:
Test cross-sign 64bits range refinement" highlights the need for a third
call to __reg_deduce_bounds. After instruction 7, reg_bounds_sync
performs the following bounds deduction:
reg_bounds_sync entry: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146)
__update_reg_bounds: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146)
__reg_deduce_bounds:
__reg32_deduce_bounds: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146,umin32=0xfffffcf1,umax32=0xffffff6e)
__reg64_deduce_bounds: scalar(smin=-655,smax=0xeffffeee,smin32=-783,smax32=-146,umin32=0xfffffcf1,umax32=0xffffff6e)
__reg_deduce_mixed_bounds: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1,umax=0xffffffffffffff6e,smin32=-783,smax32=-146,umax32=0xffffff6e)
__reg_deduce_bounds:
__reg32_deduce_bounds: scalar(smin=-655,smax=0xeffffeee,umin=umin32=0xfffffcf1,umax=0xffffffffffffff6e,smin32=-783,smax32=-146,umax32=0xffffff6e)
__reg64_deduce_bounds: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e)
__reg_deduce_mixed_bounds: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e)
__reg_bound_offset: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff))
__update_reg_bounds: scalar(smin=-655,smax=smax32=-146,umin=0xfffffffffffffd71,umax=0xffffffffffffff6e,smin32=-783,umin32=0xfffffcf1,umax32=0xffffff6e,var_off=(0xfffffffffffffc00; 0x3ff))
In particular, notice how:
1. In the first call to __reg_deduce_bounds, __reg32_deduce_bounds
learns new u32 bounds.
2. __reg64_deduce_bounds is unable to improve bounds at this point.
3. __reg_deduce_mixed_bounds derives new u64 bounds from the u32 bounds.
4. In the second call to __reg_deduce_bounds, __reg64_deduce_bounds
improves the smax and umin bounds thanks to patch "bpf: Improve
bounds when s64 crosses sign boundary" from this series.
5. Subsequent functions are unable to improve the ranges further (only
tnums). Yet, a better smin32 bound could be learned from the smin
bound.
__reg32_deduce_bounds is able to improve smin32 from smin, but for that
we need a third call to __reg_deduce_bounds.
As discussed in [1], there may be a better way to organize the deduction
rules to learn the same information with less calls to the same
functions. Such an optimization requires further analysis and is
orthogonal to the present patchset.
Link: https://lore.kernel.org/bpf/aIKtSK9LjQXB8FLY@mail.gmail.com/ [1]
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Co-developed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/79619d3b42e5525e0e174ed534b75879a5ba15de.1753695655.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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.
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