musl - Query regarding malloc if statement

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Message-ID: <CY4PR02MB2231DC947A389856977CFF4E82C40@CY4PR02MB2231.namprd02.prod.outlook.com>
Date: Mon, 19 Jun 2017 15:16:16 +0000
From: Jamie Mccrae <Jamie.Mccrae@...rdtech.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Query regarding malloc if statement

Hi,

I'm using musl to compile a cross-distro application which I've been having problems with and whilst discussing the problem the developer of another project, was shown a musl malloc function which manually checks the contents of each byte and changes it to 0 if the byte is non-0. This code is in src/malloc/malloc.c as so:

void *__malloc0(size_t n)
{
        void *p = malloc(n);
        if (p && !IS_MMAPPED(MEM_TO_CHUNK(p))) {
                size_t *z;
                n = (n + sizeof *z - 1)/sizeof *z;
                for (z=p; n; n--, z++) if (*z) *z=0;
        }
        return p;
}

This code causes thousands of errors when using valgrind (in excess of 800,000 for my application) due to checking the value of each byte before it has been set and I have to agree with this other developer that I'm at a loss as to why this is performed. If you step through the array and just set each byte to 0 then there will be no read-before-initialisation error and the function will run much faster due to not having to retrieve the data. Why not instead use:

                for (z=p; n; n--, z++) *z=0;

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