The Linux/BSD DF kernel bug
ovek at arcticnet.no
Wed Mar 19 23:54:47 CDT 2008
Francois Gouget skrev:
> The problem revolves around the x86 direction flag (DF), which
> governs whether block memory operations operate forward through
> memory or backwards. GCC [...] 4.3.0, assumes that the direction flag
> has been cleared [...] at the entry of each function, as is specified
> by the ABI [...] Prior to 4.3, GCC would emit a cld (clear direction
> flag) opcode before doing inline string or memory operations.
> So any compiler that not clear the direction flag at the start of each
> function can cause kernel crashes when running on a buggy Linux or BSD
No, it doesn't say that. The kernel is unaffected. The problem is only
with user-mode signal handlers; if DF is set when a signal occurs, the
signal handler may get confused.
> Does anyone know if MSVC does a cld in the right places?
Maybe. But it hardly matters. Only GCC-generated code is affected. The
problem might then show up in Wine signal/exception handling. Things
like, say, copying the CONTEXT structures around, for example.
In Wine, you can probably easily manually clear the flag on Wine signal
handler entry (init_handler), if you're worried.
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