SUMMARY

Both valgrind 3.15 and git (60a7ebedc0b53e2cdcea112635a285d8136b68a8) report an error. This seems similar to https://bugs.kde.org/show_bug.cgi?id=386945

Reduced from a real test cast at https://github.com/ElementsProject/lightning/issues/3672

/* Compile with -O3 */
#include <string.h>
#include <stdint.h>

int main(int argc, char *argv[])
{
	char *s = strdup("02aaa2010000000");

	for (size_t i = 0; i < strlen(s); i++) {
		if (!strncmp(s + i, " * ", 3))
			return 1;
	}
	return 0;
}


STEPS TO REPRODUCE
1. Enter the above trivial program, and compile with -O3.
2. Run valgrind ./test-valgrind


OBSERVED RESULT

==12951== Memcheck, a memory error detector
==12951== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==12951== Using Valgrind-3.16.0.GIT and LibVEX; rerun with -h for copyright info
==12951== Command: ./test-valgrind
==12951== 
==12951== Invalid read of size 4
==12951==    at 0x180774: main (in /home/rusty/test-valgrind)
==12951==  Address 0x4b1004d is 13 bytes inside a block of size 16 alloc'd
==12951==    at 0x48942AC: malloc (vg_replace_malloc.c:307)
==12951==    by 0x499C5E3: strdup (strdup.c:42)
==12951==    by 0x18071B: main (in /home/rusty/test-valgrind)
==12951== 
==12951== 
==12951== HEAP SUMMARY:
==12951==     in use at exit: 16 bytes in 1 blocks
==12951==   total heap usage: 1 allocs, 0 frees, 16 bytes allocated
==12951== 
==12951== LEAK SUMMARY:
==12951==    definitely lost: 16 bytes in 1 blocks
==12951==    indirectly lost: 0 bytes in 0 blocks
==12951==      possibly lost: 0 bytes in 0 blocks
==12951==    still reachable: 0 bytes in 0 blocks
==12951==         suppressed: 0 bytes in 0 blocks
==12951== Rerun with --leak-check=full to see details of leaked memory
==12951== 
==12951== For lists of detected and suppressed errors, rerun with: -s
==12951== ERROR SUMMARY: 2 errors from 1 contexts (suppressed: 0 from 0)




EXPECTED RESULT

No error (except the memleak, sure).

SOFTWARE/OS VERSIONS

Linux falco.openpower.xyz 4.15.0-76-generic #86-Ubuntu SMP Fri Jan 17 17:24:13 UTC 2020 ppc64le ppc64le ppc64le GNU/Linux

libc6 version: 2.27-3ubuntu1

ADDITIONAL INFORMATION

Here's the assembler from gcc -S:

	.file	"test-valgrind.c"
	.abiversion 2
	.section	".text"
	.globl strncmp
	.section	.text.startup,"ax",@progbits
	.align 2
	.p2align 4,,15
	.globl main
	.type	main, @function
main:
.LCF0:
0:	addis 2,12,.TOC.-.LCF0@ha
	addi 2,2,.TOC.-.LCF0@l
	.localentry	main,.-main
	mflr 0
	std 28,-32(1)
	std 29,-24(1)
	addis 3,2,.LC0@toc@ha
	std 30,-16(1)
	std 31,-8(1)
	addi 3,3,.LC0@toc@l
	addis 30,2,.LC1@toc@ha
	addi 30,30,.LC1@toc@l
	li 28,0
	std 0,16(1)
	stdu 1,-64(1)
	bl strdup
	nop
	mr 31,3
	bl strlen
	nop
	add 29,31,3
	b .L2
	.p2align 4,,15
.L17:
	bl strncmp
	nop
.L4:
	cmpwi 7,3,0
	addi 31,31,1
	beq 7,.L9
.L2:
	cmpld 5,31,29
	rldicl 9,31,0,52
	cmpdi 7,9,4092
	li 5,3
	mr 4,30
	mr 3,31
	beq 5,.L16
	bge 7,.L17
	lwbrx 9,0,31
	lwbrx 10,0,30
	rldicr 9,9,32,23
	rldicr 10,10,32,23
	subf. 3,10,9
	cmpb 8,9,10
	cmpb 7,9,28
	orc 8,7,8
	cntlzd 8,8
	beq 0,.L4
	addi 8,8,8
	addi 31,31,1
	rldcl 3,9,8,56
	rldcl 10,10,8,56
	subf 3,10,3
	cmpwi 7,3,0
	bne 7,.L2
.L9:
	li 3,1
	b .L7
.L16:
	li 3,0
.L7:
	addi 1,1,64
	ld 0,16(1)
	ld 28,-32(1)
	ld 29,-24(1)
	ld 30,-16(1)
	ld 31,-8(1)
	mtlr 0
	blr
	.long 0
	.byte 0,0,0,1,128,4,0,0
	.size	main,.-main
	.section	.rodata.str1.8,"aMS",@progbits,1
	.align 3
.LC0:
	.string	"02aaa2010000000"
.LC1:
	.string	" * "
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"",@progbits