On Mon, 2019-02-25 at 16:07 -0800, Linus Torvalds wrote:
> On Mon, Feb 25, 2019 at 4:03 PM Qian Cai <cai@xxxxxx> wrote:
> > >
> > > Of course, that's just gcc. I have no idea what llvm ends up doing.
> >
> > Clang 7.0:
> >
> > # clang -O2 -S -Wall /tmp/test.c
> > /tmp/test.c:46:6: warning: variable 'ret' is used uninitialized whenever
> > 'if'
> > condition is false [-Wsometimes-uninitialized]
>
> Ok, good.
>
> Do we have any clang builds in any of the zero-day robot
> infrastructure or something? Should we?
>
> And maybe this was how Dan noticed the problem in the first place? Or
> is it just because of his eagle-eyes?
>
BTW, even clang is able to generate warnings in your sample code, it does not
generate any warnings when compiling the buggy shmem.o via "make CC=clang". Here
is the objdump for arm64 (with KASAN_SW_TAGS inline).
000000000000effc <shmem_link>:
{
effc: f81c0ff7 str x23, [sp, #-64]!
f000: a90157f6 stp x22, x21, [sp, #16]
f004: a9024ff4 stp x20, x19, [sp, #32]
f008: a9037bfd stp x29, x30, [sp, #48]
f00c: 9100c3fd add x29, sp, #0x30
f010: aa0203f3 mov x19, x2
f014: aa0103f5 mov x21, x1
f018: aa0003f4 mov x20, x0
f01c: 94000000 bl 0 <_mcount>
f020: 91016280 add x0, x20, #0x58
f024: d2c20017 mov x23, #0x100000000000 //
#17592186044416
f028: b2481c08 orr x8, x0, #0xff00000000000000
f02c: f2fdfff7 movk x23, #0xefff, lsl #48
f030: d344fd08 lsr x8, x8, #4
f034: 38776909 ldrb w9, [x8, x23]
f038: 940017d5 bl 14f8c <OUTLINED_FUNCTION_11>
f03c: 54000060 b.eq f048 <shmem_link+0x4c> // b.none
f040: 7103fd1f cmp w8, #0xff
f044: 54000981 b.ne f174 <shmem_link+0x178> // b.any
f048: f9400014 ldr x20, [x0]
if (inode->i_nlink) {
f04c: 91010280 add x0, x20, #0x40
f050: b2481c08 orr x8, x0, #0xff00000000000000
f054: d344fd08 lsr x8, x8, #4
f058: 38776909 ldrb w9, [x8, x23]
f05c: 940017cc bl 14f8c <OUTLINED_FUNCTION_11>
f060: 54000060 b.eq f06c <shmem_link+0x70> // b.none
f064: 7103fd1f cmp w8, #0xff
f068: 540008a1 b.ne f17c <shmem_link+0x180> // b.any
f06c: b9400008 ldr w8, [x0]
f070: 34000148 cbz w8, f098 <shmem_link+0x9c>
f074: 940018fd bl 15468 <OUTLINED_FUNCTION_1124>
ret = shmem_reserve_inode(inode->i_sb);
f078: 38776909 ldrb w9, [x8, x23]
f07c: 940017c4 bl 14f8c <OUTLINED_FUNCTION_11>
f080: 54000060 b.eq f08c <shmem_link+0x90> // b.none
f084: 7103fd1f cmp w8, #0xff
f088: 540007e1 b.ne f184 <shmem_link+0x188> // b.any
f08c: f9400000 ldr x0, [x0]
f090: 97fffcf6 bl e468 <shmem_reserve_inode>
if (ret)
f094: 35000660 cbnz w0, f160 <shmem_link+0x164>
dir->i_size += BOGO_DIRENT_SIZE;
f098: 910122a0 add x0, x21, #0x48
f09c: b2481c08 orr x8, x0, #0xff00000000000000
f0a0: d344fd09 lsr x9, x8, #4
f0a4: 3877692a ldrb w10, [x9, x23]
f0a8: 94001828 bl 15148 <OUTLINED_FUNCTION_193>
f0ac: 54000060 b.eq f0b8 <shmem_link+0xbc> // b.none
f0b0: 7103fd1f cmp w8, #0xff
f0b4: 540006c1 b.ne f18c <shmem_link+0x190> // b.any
f0b8: 38776929 ldrb w9, [x9, x23]
f0bc: 94001a4a bl 159e4 <OUTLINED_FUNCTION_1131>
f0c0: 54000060 b.eq f0cc <shmem_link+0xd0> // b.none
f0c4: 7103fd1f cmp w8, #0xff
f0c8: 54000661 b.ne f194 <shmem_link+0x198> // b.any
f0cc: f9000009 str x9, [x0]
inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
f0d0: aa1403e0 mov x0, x20
f0d4: 910182b6 add x22, x21, #0x60
f0d8: 94000000 bl 0 <current_time>
f0dc: b2481ec9 orr x9, x22, #0xff00000000000000
f0e0: d344fd29 lsr x9, x9, #4
