回复: [Request Suggestion] How does obstack deal with the specific valid address in obstack_free ?

[TU Haoxin]

Hi Bruno,

Thanks for your insightful clarification! I have no further confusion now.

Best regards,

Haoxin

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发件人: Bruno Haible <bruno@clisp.org>
发送时间: 2022年1月11日 2:02
收件人: bug-gnulib@gnu.org <bug-gnulib@gnu.org>
抄送: TU Haoxin <haoxintu.2020@phdcs.smu.edu.sg>
主题: Re: [Request Suggestion] How does obstack deal with the specific valid address in obstack_free ?

 

Hi,

> I just have a question about the implementation intention of obstack_free (an API in obstack, which is widely used in various gnu libraries, e.g.c Glibc, more details in https://gcc.gnu.org/onlinedocs/libiberty<https://gcc.gnu.org/onlinedocs/libiberty/>)

I believe the definitive documentation of the obstack module is in
<https://www.gnu.org/software/libc/manual/html_node/Obstacks.html>
not in
<https://gcc.gnu.org/onlinedocs/libiberty/Obstacks.html>
But for your current question the answer is the same.

> The question is about the intention of how does obstack_free free an address at the bottom of a chunk in the obstack. Here is a quick demonstration code: https://godbolt.org/z/arv4ha19b
>
> My point here is that the address "string_obstack->chunk" in obstrack_free (line 40) is a valid address from this chunk, and it should be freed normally as other pointers (execute this line will crash).

The documentation says about the second argument of obstack_free:
  "If object is a null pointer, everything allocated in the obstack
   is freed. Otherwise, object must be the address of an object
   allocated in the obstack."

In my interpretation, "address of an object allocated in the obstack"
means the result of a past obstack_alloc() invocation.

Neither
  string_obstack->chunk
nor
  s+4177
are valid arguments. Therefore a crash is justified.

Bruno