Merge branch 'fix-global-subprog-verification-context'

Kumar Kartikeya Dwivedi says:

====================
Fix global subprog verification context

Global subprogs can be invoked in both sleepable and non-sleepable
contexts, but are verified based on context assumptions for the program
type as a whole and not the precise context in which they are invoked.
Nicholas identified that this can violate verifier safety assumptions.
Verify global subprogs once for each observed sleepability context and add
workqueue callback regression tests.

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20260905051224.2325381-1-memxor@gmail.com

 * Clarify that globals are checked only in contexts from which they are
   reached, with two passes only when both contexts occur. (Alexei)
 * Implement in_sleepable_context() as !in_rcu_cs(). (Alexei, Eduard)
 * Keep the original subprogram traversal order and repeat until all
   called contexts have been verified. (Eduard)
 * Explain why instruction accounting needs the saved total and why the
   exception callback uses the main program's context. (Eduard)
 * Rename the do_check_common() parameter to is_sleepable. (BPF CI)
 * Preserve the harmless global calls with __weak __noinline and check
   their instruction totals across both contexts. Drop the redundant
   task-work global RCU test and retain the review acknowledgment. (Eduard)

v1 -> v2
v1: https://lore.kernel.org/bpf/20260905034018.2095649-1-memxor@gmail.com

 * Preserve the harmless global subprog calls in the positive test with
   barrier() so LLVM cannot eliminate the intended coverage.
 * Return zero explicitly from workqueue callbacks after global subprog calls.
 * Document cumulative instruction accounting across verification contexts.
 * Fix BPF multi-line comment formatting.
====================

Link: https://patch.msgid.link/20260914131923.2544250-1-memxor@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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