Re: [PATCH] left shift of negative value do_binop src/arith_yacc.c:111:12

Harald van Dijk wrote in
 <[email protected]>:
 |On 20/08/2025 01:33, Aleksander Ushakov wrote:
 |> Well, if we take bash with UBSAN instrumentation and try to use a 
 |> similar command with a negative number shift to the left, then at least 
 |> we won't get a sanitizer error. Yes, you can remove the message that 
 |> this shift is invalid, but it is still UB to perform this shift 
 |> according to the C standard. I mean, a check should be added here so as 
 |> not to trigger UBSAN, as it is done in bash.
 |
 |I did that in my own version back in 2019, with the exception of subtly 
 |changing behaviour that differed across architectures, and changing 
 |behaviour that resulted in shell crashes: 
 |<https://github.com/hvdijk/gwsh/commit/3b2307005d9ca18f57151ba239c3a0c3c\
 |107d681> 
 |and 
 |<https://github.com/hvdijk/gwsh/commit/c4d6f1b1613c228db1e157a2136c53f55\
 |b4530c8>. 
 |As those commits show, there are quite a few more cases to consider than 
 |left shifts of negative values.

We (i am only a MUA though, since busybox did not take the code,
and kept their broken implemenation; will try again at some time)
all return -2 for "-1 << 1", at least on this hardware etc etc.
You then return 0 from looking at the commitdiff!?!

 |dash is free to take those patches, or to decide not to: you are right 
 |that these cases are undefined according to the C standard, but that 
 |means that these cases are also undefined in shell scripts, because 
 |shell arithmetic is defined by reference to the C standard: "The 
 |evaluation of arithmetic expressions shall be equivalent to that 
 |described in Section 6.5, Expressions, of the ISO C standard." The 
 |current behaviour of tolerating it by default, but flagging it when 
 |special options are used, is not my preferred behaviour, but is 
 |defensible, so I will wait for dash's maintainer to chime in on his 
 |preference.
 --End of <[email protected]>

--steffen
|
|Der Kragenbaer,                The moon bear,
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