Son Tuan VU via llvm-dev
2018-Apr-27 14:48 UTC
[llvm-dev] [DbgInfo] Potential bug in location list address ranges
Hi all,
Consider this ARM assembly code of a C function:
00008124 <foo>:
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 <End of list>
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,
Son Tuan Vu
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Adrian Prantl via llvm-dev
2018-Apr-27 15:22 UTC
[llvm-dev] [DbgInfo] Potential bug in location list address ranges
> On Apr 27, 2018, at 7:48 AM, Son Tuan VU <sontuan.vu119 at gmail.com> wrote: > > Hi all, > > Consider this ARM assembly code of a C function: > > 00008124 <foo>: > 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 <End of list> > > 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> > 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, > > Son Tuan Vu-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180427/b7d9414f/attachment.html>
via llvm-dev
2018-Apr-27 15:53 UTC
[llvm-dev] [DbgInfo] Potential bug in location list address ranges
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
From: aprantl at apple.com [mailto:aprantl at apple.com]
Sent: Friday, April 27, 2018 11:22 AM
To: Son Tuan VU
Cc: Robinson, Paul; Vedant Kumar; dblaikie at gmail.com; llvm-dev
Subject: Re: [DbgInfo] Potential bug in location list address ranges
On Apr 27, 2018, at 7:48 AM, Son Tuan VU <sontuan.vu119 at
gmail.com<mailto:sontuan.vu119 at gmail.com>> wrote:
Hi all,
Consider this ARM assembly code of a C function:
00008124 <foo>:
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 <End of list>
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
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,
Son Tuan Vu
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