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Beuc
Joined: 26 Mar 2009
Posts: 33
Location: holland
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 Posted: Wed Apr 08, 2009 6:36 pm Post subject: [Solved] Pointer dereference crash while valid ptr (zziplib) |
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Hi,
I got a weird zziplib crash and I don't really understand what is going wrong %)
It's in zzip.c, line 385 (I debugged with 'gcc -g' and psplink). A pointer representing a zip file entry is dereferenced, and the PSP crashes ("Program received signal SIGHUP, Hangup.").
I tried to decompose the code, and then rewrote it using bit shifts - and then it worked.
Do you know what went wrong here?
Here are my attempts:
| Code: |
/* Original:
hdr->d_off = zzip_disk_entry_get_offset (d);
*/
/* After macro processing:
hdr->d_off = (*(uint32_t*)((d)->z_offset));
*/
/*
Decompose:
uint32_t* t_pointer = (uint32_t*)d->z_offset;
uint32_t t_value = *t_pointer; // crashes here
hdr->d_off = t_value;
*/
/* Doesn't crash: */
char* t_pointer2 = d->z_offset;
unsigned char t_value0 = t_pointer2[0];
unsigned char t_value1 = t_pointer2[1];
unsigned char t_value2 = t_pointer2[2];
unsigned char t_value3 = t_pointer2[3];
hdr->d_off = (t_value3 << 24) | (t_value2 << 16) | (t_value1 << 8) | (t_value0);
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Last edited by Beuc on Fri Apr 17, 2009 6:22 am; edited 1 time in total |
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jbit
Site Admin
Joined: 28 May 2005
Posts: 293
Location: København, Danmark
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| Posted: Wed Apr 08, 2009 6:42 pm Post subject: |
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My random guess is that d->z_offset isn't always four bytes aligned? Which is a real issue on most RISC machines ;)
The original is doing a 32bit load which needs the pointer to be aligned to 4bytes, and your "doesn't crash" code is doing 8bit loads which don't care. |
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Beuc
Joined: 26 Mar 2009
Posts: 33
Location: holland
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| Posted: Wed Apr 08, 2009 7:48 pm Post subject: |
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This could well be the reason, the field is at offset 42 in the structure.
It didn't crash when I did it manually from GDB though!
I'm surprised it works for other devs. Admittedly my .zip is special as it's appened to EBOOT.PBP (with zip -A), in an attempt to create bundled resources, so it has a non-zero offset :) However the code that parses d->z_offset is always executed AFAICS.
I'm going to submit a patch for this; do you think my solution is fine, or is there a better way? |
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Beuc
Joined: 26 Mar 2009
Posts: 33
Location: holland
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