Re: Help understanding some valgrind output

On 6/4/2020 7:26 PM, James Read wrote:
> Here is my valgrind output that I don't understand:
>
> ==319842== Invalid write of size 1
> ==319842==    at 0x48436E4: mempcpy (in 
> /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
> ==319842==    by 0x50CD1D8: _IO_default_xsputn (genops.c:386)
> ==319842==    by 0x50CD1D8: _IO_default_xsputn (genops.c:370)
> ==319842==    by 0x50B227B: __vfprintf_internal (vfprintf-internal.c:1688)
> ==319842==    by 0x50C0278: __vsprintf_internal (iovsprintf.c:95)
> ==319842==    by 0x509D047: sprintf (sprintf.c:30)
> ==319842==    by 0x10B88F: html_link_find (crawler.c:452)
> ==319842==    by 0x10BD6F: html_parse (crawler.c:536)
> ==319842==    by 0x10C2CB: check_multi_info (crawler.c:678)
> ==319842==    by 0x10C3DA: event_cb (crawler.c:706)
> ==319842==    by 0x10D828: crawler_init (crawler.c:1154)
> ==319842==    by 0x10DAE8: main (crawler.c:1207)
> ==319842==  Address 0xf107d18 is 0 bytes after a block of size 8,200 
> alloc'd
> ==319842==    at 0x483B7F3: malloc (in 
> /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
> ==319842==    by 0x10B736: html_link_find (crawler.c:440)
> ==319842==    by 0x10BD6F: html_parse (crawler.c:536)
> ==319842==    by 0x10C2CB: check_multi_info (crawler.c:678)
> ==319842==    by 0x10C3DA: event_cb (crawler.c:706)
> ==319842==    by 0x10D828: crawler_init (crawler.c:1154)
> ==319842==    by 0x10DAE8: main (crawler.c:1207)
> ==319842==
>
> valgrind: m_mallocfree.c:305 (get_bszB_as_is): Assertion 'bszB_lo == 
> bszB_hi' failed.
> valgrind: Heap block lo/hi size mismatch: lo = 8272, hi = 
> 3625731377157460067.
> This is probably caused by your program erroneously writing past the
> end of a heap block and corrupting heap metadata.  If you fix any
> invalid writes reported by Memcheck, this assertion failure will
> probably go away.  Please try that before reporting this as a bug.
>
> The code this pertains to can be found at 
> https://github.com/JamesRead5737/webcrawler
>
> Any help in understanding what this error means would be greatly 
> appreciated.
>
> James Read
>

Line 452 of crawler.c (routine html_link_find()) is calling sprintf(), 
which is then writing past the end of an allocated buffer of 8200 
bytes.  My first guess is that you are not resizing the buffer enough to 
fit the text being written by sprintf().

Find out why this is occurring in html_link_find() (it should happen 
even when your code is not running under valgrind), fix it, then rerun 
valgrind.  If your program is writing outside of buffers, it is possible 
that valgrind won't catch it and thus the assertion failure later.

-- 
     David Chapman      [email protected]
     Chapman Consulting -- San Jose, CA
     EDA Software Developer, Expert Witness
     www.chapman-consulting-sj.com
     2018-2019 Chair, IEEE Consultants' Network of Silicon Valley



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