[ added a few CCs ]

Hello Dave,

On 5/5/21 10:30 PM, Dave Chupreev wrote:

Hello, Alejandro.
 
On page 138

I guess you refer to TLPI, written by Michael.

     1.

        Your version of /unsetenv() /should check to see whether there
        are multiple definitions of an environment variable, and remove
        them all.

How can I add such variables which have many definitions? According to
*putenv* and *setenv* functions, variables with the same names are
replaced if encountered.

I haven't read that part of the book yet, so I ignore the context.  But
AFAIK, that can't happen on Linux, as you pointed out (probably neither
on Unix systems in general, but I don't know for sure, probably Michael
does).  I guess the only possibility is if an attacker somehow modified
your environment and inserted multiple copies of an env variable.

The book (TLPI) states that glibc does check that, so I digged into the
sources and found that in <stdlib/setenv.c>, around line 290
(<https://sourceware.org/git?p=glibc.git;a=blob;f=stdlib/setenv.c;h=893f081af6b5a21b999a4056757fd69d1386c0d4;hb=HEAD#l290>).
 That behavior was introduced by Roland in commit
196980f5117c8d38f10d64bf67eeb0924651675f
(<https://sourceware.org/git/?p=glibc.git;a=commit;h=196980f5117c8d38f10d64bf67eeb0924651675f>),
so maybe he can better explain the reasons behind the change (the commit
msg is quite unexplicative) if he still remembers (that goes back to 1995).

Regards,

Alex

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/