[9fans] Why getenv replaces \0 with spaces in the returned value?

[9fans] Why getenv replaces \0 with spaces in the returned value?

[Giacomo Tesio]

Hi, last night I noticed this strange post processing in 4th edition's
getenv: https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/getenv.c#L34-L41

        seek(f, 0, 0);
        r = read(f, ans, s);
        if(r >= 0) {
            ep = ans + s - 1;
            for(p = ans; p < ep; p++)
                if(*p == '\0')
                    *p = ' ';
            ans[s] = '\0';
        }

Anybody know why this replacement is done?
It does not seem a good fix to read/write or read/truncate races, but
I can't find a better explanation.


Giacomo

Re: [9fans] Why getenv replaces \0 with spaces in the returned value?

[Ryan Gonzalez]

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Just a wild guess, but I think it could be because getenv just returns a null-
terminated string with no indication of its length. If C code were to do
pretty much anything on the environment variable in question, it would always
be truncated. e.g. with VAR=ABC\0DEF, the C string processing functions would
assume it ended after ABC.

  

As a side note, Linux does nothing and simply returns it as-is. I wonder why
the Posix guys haven't added a getenv_n(var, &length) function yet...

  

\--  

Ryan (ライアン)

Yoko Shimomura > ryo (supercell/EGOIST) > Hiroyuki Sawano >> everyone else

<http://refi64.com/>

  
On Jan 18 2017, at 10:47 am, Giacomo Tesio <giacomo@tesio.it> wrote:  

> Hi, last night I noticed this strange post processing in 4th edition's  
getenv:
https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/getenv.c#L34-L41

>

>         seek(f, 0, 0);  
        r = read(f, ans, s);  
        if(r >= 0) {  
            ep = ans + s - 1;  
            for(p = ans; p < ep; p++)  
                if(*p == '\0')  
                    *p = ' ';  
            ans[s] = '\0';  
        }

>

> Anybody know why this replacement is done?  
It does not seem a good fix to read/write or read/truncate races, but  
I can't find a better explanation.

>

>  
Giacomo


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Re: [9fans] Why getenv replaces \0 with spaces in the returned value?

[Fran. J Ballesteros]

rc lists?

> El 18 ene 2017, a las 17:45, Giacomo Tesio <giacomo@tesio.it> escribió:
> 
> Hi, last night I noticed this strange post processing in 4th edition's
> getenv: https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/getenv.c#L34-L41
> 
>        seek(f, 0, 0);
>        r = read(f, ans, s);
>        if(r >= 0) {
>            ep = ans + s - 1;
>            for(p = ans; p < ep; p++)
>                if(*p == '\0')
>                    *p = ' ';
>            ans[s] = '\0';
>        }
> 
> Anybody know why this replacement is done?
> It does not seem a good fix to read/write or read/truncate races, but
> I can't find a better explanation.
> 
> 
> Giacomo
> 

Re: [9fans] Why getenv replaces \0 with spaces in the returned value?

[Charles Forsyth]

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Yes, it's the lists. Nothing will cope with \0 in a C string, so it's a
good choice as list of string element separator.

On 18 January 2017 at 19:21, Fran. J Ballesteros <nemo@lsub.org> wrote:

> rc lists?
>
> > El 18 ene 2017, a las 17:45, Giacomo Tesio <giacomo@tesio.it> escribió:
> >
> > Hi, last night I noticed this strange post processing in 4th edition's
> > getenv: https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/
> getenv.c#L34-L41
> >
> >        seek(f, 0, 0);
> >        r = read(f, ans, s);
> >        if(r >= 0) {
> >            ep = ans + s - 1;
> >            for(p = ans; p < ep; p++)
> >                if(*p == '\0')
> >                    *p = ' ';
> >            ans[s] = '\0';
> >        }
> >
> > Anybody know why this replacement is done?
> > It does not seem a good fix to read/write or read/truncate races, but
> > I can't find a better explanation.
> >
> >
> > Giacomo
> >
>
>
>

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Re: [9fans] Why getenv replaces \0 with spaces in the returned value?

[Giacomo Tesio]

Yes that would be a plausible explanation but actually rc does not use
getenv: it reads /env/ files directly.

I've tried to remove the loop and I can't see any issue.


Giacomo


2017-01-18 21:13 GMT+01:00 Charles Forsyth <charles.forsyth@gmail.com>:
> Yes, it's the lists. Nothing will cope with \0 in a C string, so it's a good
> choice as list of string element separator.
>
> On 18 January 2017 at 19:21, Fran. J Ballesteros <nemo@lsub.org> wrote:
>>
>> rc lists?
>>
>> > El 18 ene 2017, a las 17:45, Giacomo Tesio <giacomo@tesio.it> escribió:
>> >
>> > Hi, last night I noticed this strange post processing in 4th edition's
>> > getenv:
>> > https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/getenv.c#L34-L41
>> >
>> >        seek(f, 0, 0);
>> >        r = read(f, ans, s);
>> >        if(r >= 0) {
>> >            ep = ans + s - 1;
>> >            for(p = ans; p < ep; p++)
>> >                if(*p == '\0')
>> >                    *p = ' ';
>> >            ans[s] = '\0';
>> >        }
>> >
>> > Anybody know why this replacement is done?
>> > It does not seem a good fix to read/write or read/truncate races, but
>> > I can't find a better explanation.
>> >
>> >
>> > Giacomo
>> >
>>
>>
>

Re: [9fans] Why getenv replaces \0 with spaces in the returned value?

[Charles Forsyth]

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Yes but other programs might like to read values left behind by rc, using
getenv

On Wed, 18 Jan 2017, 23:51 Giacomo Tesio, <giacomo@tesio.it> wrote:

> Yes that would be a plausible explanation but actually rc does not use
> getenv: it reads /env/ files directly.
>
> I've tried to remove the loop and I can't see any issue.
>
>
> Giacomo
>
>
> 2017-01-18 21:13 GMT+01:00 Charles Forsyth <charles.forsyth@gmail.com>:
> > Yes, it's the lists. Nothing will cope with \0 in a C string, so it's a
> good
> > choice as list of string element separator.
> >
> > On 18 January 2017 at 19:21, Fran. J Ballesteros <nemo@lsub.org> wrote:
> >>
> >> rc lists?
> >>
> >> > El 18 ene 2017, a las 17:45, Giacomo Tesio <giacomo@tesio.it>
> escribió:
> >> >
> >> > Hi, last night I noticed this strange post processing in 4th edition's
> >> > getenv:
> >> >
> https://github.com/brho/plan9/blob/master/sys/src/libc/9sys/getenv.c#L34-L41
> >> >
> >> >        seek(f, 0, 0);
> >> >        r = read(f, ans, s);
> >> >        if(r >= 0) {
> >> >            ep = ans + s - 1;
> >> >            for(p = ans; p < ep; p++)
> >> >                if(*p == '\0')
> >> >                    *p = ' ';
> >> >            ans[s] = '\0';
> >> >        }
> >> >
> >> > Anybody know why this replacement is done?
> >> > It does not seem a good fix to read/write or read/truncate races, but
> >> > I can't find a better explanation.
> >> >
> >> >
> >> > Giacomo
> >> >
> >>
> >>
> >
>
>

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