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* [Bug malloc/29283] New: assert when ptrace(PTRACE_POKEDATA) on allocated buffer
@ 2022-06-24 1:26 russkubik at gmail dot com
2022-06-24 1:26 ` [Bug malloc/29283] " russkubik at gmail dot com
2022-06-24 7:22 ` fweimer at redhat dot com
0 siblings, 2 replies; 3+ messages in thread
From: russkubik at gmail dot com @ 2022-06-24 1:26 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29283
Bug ID: 29283
Summary: assert when ptrace(PTRACE_POKEDATA) on allocated
buffer
Product: glibc
Version: 2.35
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: malloc
Assignee: unassigned at sourceware dot org
Reporter: russkubik at gmail dot com
Target Milestone: ---
Created attachment 14165
--> https://sourceware.org/bugzilla/attachment.cgi?id=14165&action=edit
tracer.c
I am seeing an assert when a program that is set up to ptrace a child uses
ptrace(PTRACE_POKEDATA) to modify the contents of the child process' malloc
allocated buffer.
> tracee: malloc.c:2617: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.
This only seems to occur if the buffer size that the tracee program requests
from malloc matches the usable size reported by malloc_usable_size.
Reproducer (x86-64 specific):
```
$ gcc -o tracee tracee.c
$ gcc -o tracer tracer.c
```
Locate the starting heap address for programs when address randomization is
disabled and provide that as an argument to the tracer program.
When requested size == malloc usable size, an assert:
```
$ BUFSIZ=72 ./tracer 0x5555555592a0 ./tracee
tracee: malloc.c:2617: sysmalloc: Assertion `(old_top == initial_top (av) &&
old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse
(old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.
$ BUFSIZ=80 ./tracer 0x5555555592a0 ./tracee
buf: 0x5555555592a0
tracee: malloc.c:2617: sysmalloc: Assertion `(old_top == initial_top (av) &&
old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse
(old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.
```
When requested size != malloc usable size (ie. we have padding at the end), all
is well:
```
$ BUFSIZ=71 ./tracer 0x5555555592a0 ./tracee
buf: 0x5555555592a0
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA000000
$ BUFSIZ=73 ./tracer 0x5555555592a0 ./tracee
buf: 0x5555555592a0
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA00
```
This does not appear to be an issue with jemalloc (used as a sanity test):
```
$ BUFSIZ=72 LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so ./tracer
0x7ffff721e000 ./tracee
buf: 0x7ffff721e000
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
```
Any insight appreciated, thx.
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2022-06-24 1:26 [Bug malloc/29283] New: assert when ptrace(PTRACE_POKEDATA) on allocated buffer russkubik at gmail dot com
2022-06-24 1:26 ` [Bug malloc/29283] " russkubik at gmail dot com
2022-06-24 7:22 ` fweimer at redhat dot com
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