[DbgInfo] Potential bug in location list address ranges

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1

Hi all,

Consider this ARM assembly code of a C function:

00008124 :
8124: push {r4, r6, r7, lr}
8126: add r7, sp, #8
8128: mov r4, r0
812a: ldrsb.w r0, [r2]
812e: cmp r0, #1
8130: itt lt
8132: movlt r0, #85 ; 0x55
8134: poplt {r4, r6, r7, pc} // a function return

8136: ldrb.w ip, [r1, #3]
813a: ldrb.w lr, [r4, #3]
813e: movs r0, #85 ; 0x55
8140: cmp lr, ip
8142: bne.n 8168 <foo+0x44>

8144: ldrb.w ip, [r1, #2]
8148: ldrb r3, [r4, #2]
814a: cmp r3, ip
814c: it ne
814e: popne {r4, r6, r7, pc} // a function return

8150: ldrb.w ip, [r1, #1]
8154: ldrb r3, [r4, #1]
8156: cmp r3, ip
8158: bne.n 8168 <foo+0x44>

815a: ldrb r1, [r1, #0]
815c: ldrb r3, [r4, #0]
815e: cmp r3, r1
8160: ittt eq
8162: moveq r0, #3
8164: strbeq r0, [r2, #0]
8166: moveq r0, #170 ; 0xaa
8168: pop {r4, r6, r7, pc} // a function return

I have a variable bar and here’s its corresponding DWARF DIE:

<2><3b>: Abbrev Number: 3 (DW_TAG_formal_parameter)
<3c> DW_AT_location : 0x0 (location list)
<40> DW_AT_name : (indirect string, offset: 0x9e): bar
<44> DW_AT_decl_file : 1
<45> DW_AT_decl_line : 34
<46> DW_AT_type : <0x153>

// Its location list

00000000 00008124 0000812a (DW_OP_reg0 (r0))
0000000b 0000812a 00008136 (DW_OP_reg4 (r4))
00000016

As you can see, it says that we can find bar in r4 from 0x812a to 0x8134 (poplt). However, this is only true when the cmp instruction at 0x812e yields less than (lt). So if the value in r0 is greater than 1 (which is the case of my input), we should still be able to read the value of bar from r4 in the remaining of the function.

I don’t know if we can consider this a bug, because I don’t even know what should be the correct location information for bar. However, in this case, since the conditional instruction that clobbers r4 is a function return, I’d expect to read the value of bar from r4 in the remaining of the function.

If the conditional instruction poplt was addlt r4, r0, 3 for example, what should be the correct location list of bar?

For now, my only idea is to check if the clobbering MI is a conditional return in DbgValueHistoryCalculator which computes the end address of a location llist entry. But I do not feel like this is the correct fix though.

Looking forward to hearing your thoughts on this,

Thank you for reading this,

2

Hi all,

Consider this ARM assembly code of a C function:

00008124 :
8124: push {r4, r6, r7, lr}
8126: add r7, sp, #8
8128: mov r4, r0
812a: ldrsb.w r0, [r2]
812e: cmp r0, #1
8130: itt lt
8132: movlt r0, #85 ; 0x55
8134: poplt {r4, r6, r7, pc} // a function return

8136: ldrb.w ip, [r1, #3]
813a: ldrb.w lr, [r4, #3]
813e: movs r0, #85 ; 0x55
8140: cmp lr, ip
8142: bne.n 8168 <foo+0x44>

8144: ldrb.w ip, [r1, #2]
8148: ldrb r3, [r4, #2]
814a: cmp r3, ip
814c: it ne
814e: popne {r4, r6, r7, pc} // a function return

8150: ldrb.w ip, [r1, #1]
8154: ldrb r3, [r4, #1]
8156: cmp r3, ip
8158: bne.n 8168 <foo+0x44>

815a: ldrb r1, [r1, #0]
815c: ldrb r3, [r4, #0]
815e: cmp r3, r1
8160: ittt eq
8162: moveq r0, #3
8164: strbeq r0, [r2, #0]
8166: moveq r0, #170 ; 0xaa
8168: pop {r4, r6, r7, pc} // a function return

I have a variable bar and here’s its corresponding DWARF DIE:

<2><3b>: Abbrev Number: 3 (DW_TAG_formal_parameter)
<3c> DW_AT_location : 0x0 (location list)
<40> DW_AT_name : (indirect string, offset: 0x9e): bar
<44> DW_AT_decl_file : 1
<45> DW_AT_decl_line : 34
<46> DW_AT_type : <0x153>

// Its location list

00000000 00008124 0000812a (DW_OP_reg0 (r0))
0000000b 0000812a 00008136 (DW_OP_reg4 (r4))
00000016

As you can see, it says that we can find bar in r4 from 0x812a to 0x8134 (poplt). However, this is only true when the cmp instruction at 0x812e yields less than (lt). So if the value in r0 is greater than 1 (which is the case of my input), we should still be able to read the value of bar from r4 in the remaining of the function.

I don’t know if we can consider this a bug, because I don’t even know what should be the correct location information for bar. However, in this case, since the conditional instruction that clobbers r4 is a function return, I’d expect to read the value of bar from r4 in the remaining of the function.

I can’t tell for sure whether the debug info is correct without also seeing the source code, but as a general point: Debug information is must-information that holds over all paths through the program. Debug information that is only accurate for some paths is a bug. A serious bug, because if the user can’t rely on the debug info to be correct in some cases, they can’t rely on any of the debug info to be correct.

– adrian

3

As Adrian said, we’d need to see the source of foo() to assess what the location-list for bar ought to be.

Without actually going to look, I would guess that ‘poplt’ is considered a conditional move, therefore r4’s contents are not guaranteed after it executes (i.e. it is a clobber). If one operand of ‘poplt’ is ‘pc’ then of course it is also a conditional indirect branch (which is probably but not necessarily a return). This combination might be worth handling differently for location-list purposes.

But this is a tricky area, and we’d need to consider the consequences carefully.

–paulr

4

Thank you all for taking a look at this. I pasted the C source then deleted it because I was afraid that it was too long to read…

Here’s the code of foo. Its real name is verifyPIN. The variable bar is userPin.

int verifyPIN(char *userPin, char *cardPin, int *cpt)
{
int i;
int status;
int diff;

if (*cpt > 0) {
status = 0x55;
diff = 0x55;

for (i = 0; i < 4; i++) {
if (userPin[i] != cardPin[i]) {
diff = 0xAA;
}
}

if (diff == 0x55) {
status = 0xAA;
}
else {
status = 0x55;
}

if (status == 0xAA) {
*cpt = 3;
return 0xAA;
} else {
*cpt–;
return 0x55;
}
}

return 0x55;
}

@paul: Yes you are right, I have investigated the backend and it all starts at IfConversionPass. r4 is clobbered by poplt, and there’s no logic to handle conditional instruction in DbgValueHistoryCalculator, thus the issue at the binary level.

5

Hello,

Has anyone taken a look at this bug? I really want to fix this, but as Paul pointed out, this requires a lot of care…

Thank you for your help

6

Could you file a bug report about this (bugs.llvm.org)? If you don’t have an account on bugzilla, I’d be happy to file one for you. Please provide exact instructions to reproduce the issue including any compilation flags.

thanks,
vedant

7

Hello,

This is the bug report related to this thread https://bugs.llvm.org/show_bug.cgi?id=37391. I didn’t receive an e-mail for creating the bug so I post it here just in case you miss the notification.

Thanks,