[Bug stdio/18375] New: Syscall param write(buf) points to uninitialised byte(s) (valgrind reports)

[Bug stdio/18375] New: Syscall param write(buf) points to uninitialised byte(s) (valgrind reports)

[eugeniofonseca14 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18375

            Bug ID: 18375
           Summary: Syscall param write(buf) points to uninitialised
                    byte(s) (valgrind reports)
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: eugeniofonseca14 at gmail dot com
  Target Milestone: ---

Created attachment 8300
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8300&action=edit
Code that reproduces the error.

Hello all,

The code I'm sending generates a valid output file, but valgrind accuses
fclose() of violating memory, when flushing the file buffer to disk. fcloses
does what it is supposed to do, despite the memory access error.

#include<stdio.h>
#include<stdlib.h>

typedef struct{
    int x;
    int y;
    double distanceFromBase;
    unsigned int tanks;
    long long int ackIndex;
} TankAttack;


int main(){
    FILE* arq;
    TankAttack* p = (TankAttack*)malloc(sizeof(TankAttack));

    arq = fopen("0","wb");

    p->x = 10;
    p->y = 10;
    p->distanceFromBase = 10.0;
    p->tanks = 10;
    p->ackIndex = 10;

    fwrite(p,sizeof(TankAttack),1,arq);

    fclose(arq);
    free(p);
    return 0;
}

==6143== Syscall param write(buf) points to uninitialised byte(s)
==6143==    at 0x4F2F4E0: __write_nocancel (syscall-template.S:81)
==6143==    by 0x4EB1BFE: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1251)
==6143==    by 0x4EB30E8: new_do_write (fileops.c:506)
==6143==    by 0x4EB30E8: _IO_do_write@@GLIBC_2.2.5 (fileops.c:482)
==6143==    by 0x4EB296F: _IO_file_close_it@@GLIBC_2.2.5 (fileops.c:164)
==6143==    by 0x4EA6851: fclose@@GLIBC_2.2.5 (iofclose.c:59)
==6143==    by 0x4006E5: main (in /home/eugenio/AEDS/WW2/out)
==6143==  Address 0x4026014 is not stack'd, malloc'd or (recently) free'd
==6143== 
==6143== 
==6143== HEAP SUMMARY:
==6143==     in use at exit: 0 bytes in 0 blocks
==6143==   total heap usage: 2 allocs, 2 frees, 584 bytes allocated
==6143== 
==6143== All heap blocks were freed -- no leaks are possible

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[Bug stdio/18375] Syscall param write(buf) points to uninitialised byte(s) (valgrind reports)

[eugeniofonseca14 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18375

--- Comment #1 from Eugênio Fonseca <eugeniofonseca14 at gmail dot com> ---
Command lines used:

gcc teste.c -o out
valgrind ./out

GCC version:
gcc (Ubuntu 4.9.2-10ubuntu13) 4.9.2

Valgrind version:
valgrind-3.10.1

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Subject: [Bug stdio/18375] Syscall param write(buf) points to uninitialised byte(s) (valgrind reports)
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https://sourceware.org/bugzilla/show_bug.cgi?id=18375

Eugênio Fonseca <eugeniofonseca14 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unspecified                 |2.21

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https://sourceware.org/bugzilla/show_bug.cgi?id\x15593

Mike FABIAN <maiku.fabian at gmail dot com> changed:

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                 CC|                            |maiku.fabian at gmail dot com

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[Bug stdio/18375] Syscall param write(buf) points to uninitialised byte(s) (valgrind reports)

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18375

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
>     fwrite(p,sizeof(TankAttack),1,arq);

You are passing a (partly) uninitialized memory block to fwrite.

This is really a bug in valgrind, since it doesn't know about struct padding.

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