[Bug libc/15854] New: strtod should avoid calling strlen

[Bug libc/15854] New: strtod should avoid calling strlen

[emogenet at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

            Bug ID: 15854
           Summary: strtod should avoid calling strlen
           Product: glibc
           Version: 2.18
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: emogenet at gmail dot com
                CC: drepper.fsp at gmail dot com

Problem : glibc's strtod seem to systematically call strlen on its input.

To the layman that I am, there doesn't seem to be any legitimate reason why it
should: it seems that strtod should simply consume its input one char at a time
until it reaches a char that marks the end of a valid FP number ASCII rep. and
should therefore work on a non-zero terminated buffer, as long said buffer ends
with a char that terminates the parsing.

This internal call to strlen makes it essentially impossible to call strtod
on a no zero terminated buffer, and there seems to be no other way to otherwise
access the non-trivial code that converts an ASCII buffer to a FP number.

This makes it in particular painful to call strtod on a very large mmap'd
buffer of ASCII floats : strlen will plow through the entire file for every
call to strtod, making things highly inefficient (it is also not guaranteed
not to crash).

To work around this shortcoming, one ends up having to figure out the end of
the FP ASCII string, "by hand", copy the result to a zero terminated buffer,
and then call strtod on that.

This is both inefficient and clunky.

See this article for a good description of the issue:

http://www.ryanjuckett.com/programming/c-cplusplus/25-optimizing-atof-and-strtod

Here's another instance of the problem:

http://stackoverflow.com/questions/2033845/any-one-know-how-to-convert-a-huge-char-array-to-float-very-huge-array-perform

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[Bug libc/15854] strtod should avoid calling strlen

[emogenet at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

emogenet at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |emogenet at gmail dot com

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Re: [Bug libc/15854] New: strtod should avoid calling strlen

[Ondřej Bílka]

On Tue, Aug 20, 2013 at 02:12:32AM +0000, emogenet at gmail dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
> 
>             Bug ID: 15854
>            Summary: strtod should avoid calling strlen
>            Product: glibc
>            Version: 2.18
>             Status: NEW
>           Severity: enhancement
>           Priority: P2
>          Component: libc
>           Assignee: unassigned at sourceware dot org
>           Reporter: emogenet at gmail dot com
>                 CC: drepper.fsp at gmail dot com
> 
> Problem : glibc's strtod seem to systematically call strlen on its input.
> 
> To the layman that I am, there doesn't seem to be any legitimate reason why it
> should: it seems that strtod should simply consume its input one char at a time
> until it reaches a char that marks the end of a valid FP number ASCII rep. and
> should therefore work on a non-zero terminated buffer, as long said buffer ends
> with a char that terminates the parsing.
>
This is not that big problem, strtod only uses strlen in following context

  decimal = _NL_CURRENT (LC_NUMERIC, DECIMAL_POINT); // which is "."
  decimal_len = strlen (decimal); // which is 1

 
> This internal call to strlen makes it essentially impossible to call strtod
> on a no zero terminated buffer, and there seems to be no other way to otherwise
> access the non-trivial code that converts an ASCII buffer to a FP number.
> 
> This makes it in particular painful to call strtod on a very large mmap'd
> buffer of ASCII floats : strlen will plow through the entire file for every
> call to strtod, making things highly inefficient (it is also not guaranteed
> not to crash).
>
Do you have testcase to demonstrate quadratic behavior? It is possible
that end is determined by other ineffective means.
 
> To work around this shortcoming, one ends up having to figure out the end of
> the FP ASCII string, "by hand", copy the result to a zero terminated buffer,
> and then call strtod on that.
> 
> This is both inefficient and clunky.
> 
> See this article for a good description of the issue:
> 
> http://www.ryanjuckett.com/programming/c-cplusplus/25-optimizing-atof-and-strtod
> 
> Here's another instance of the problem:
> 
> http://stackoverflow.com/questions/2033845/any-one-know-how-to-convert-a-huge-char-array-to-float-very-huge-array-perform
> 
Not relevant for us as these are windows problems.

[Bug libc/15854] strtod should avoid calling strlen

[neleai at seznam dot cz]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

--- Comment #1 from Ondrej Bilka <neleai at seznam dot cz> ---
On Tue, Aug 20, 2013 at 02:12:32AM +0000, emogenet at gmail dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
> 
>             Bug ID: 15854
>            Summary: strtod should avoid calling strlen
>            Product: glibc
>            Version: 2.18
>             Status: NEW
>           Severity: enhancement
>           Priority: P2
>          Component: libc
>           Assignee: unassigned at sourceware dot org
>           Reporter: emogenet at gmail dot com
>                 CC: drepper.fsp at gmail dot com
> 
> Problem : glibc's strtod seem to systematically call strlen on its input.
> 
> To the layman that I am, there doesn't seem to be any legitimate reason why it
> should: it seems that strtod should simply consume its input one char at a time
> until it reaches a char that marks the end of a valid FP number ASCII rep. and
> should therefore work on a non-zero terminated buffer, as long said buffer ends
> with a char that terminates the parsing.
>
This is not that big problem, strtod only uses strlen in following context

  decimal = _NL_CURRENT (LC_NUMERIC, DECIMAL_POINT); // which is "."
  decimal_len = strlen (decimal); // which is 1


> This internal call to strlen makes it essentially impossible to call strtod
> on a no zero terminated buffer, and there seems to be no other way to otherwise
> access the non-trivial code that converts an ASCII buffer to a FP number.
> 
> This makes it in particular painful to call strtod on a very large mmap'd
> buffer of ASCII floats : strlen will plow through the entire file for every
> call to strtod, making things highly inefficient (it is also not guaranteed
> not to crash).
>
Do you have testcase to demonstrate quadratic behavior? It is possible
that end is determined by other ineffective means.

> To work around this shortcoming, one ends up having to figure out the end of
> the FP ASCII string, "by hand", copy the result to a zero terminated buffer,
> and then call strtod on that.
> 
> This is both inefficient and clunky.
> 
> See this article for a good description of the issue:
> 
> http://www.ryanjuckett.com/programming/c-cplusplus/25-optimizing-atof-and-strtod
> 
> Here's another instance of the problem:
> 
> http://stackoverflow.com/questions/2033845/any-one-know-how-to-convert-a-huge-char-array-to-float-very-huge-array-perform
> 
Not relevant for us as these are windows problems.

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[Bug libc/15854] strtod should avoid calling strlen

[emogenet at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

--- Comment #2 from emogenet at gmail dot com ---
You are correct, the call tp strlen in strtod is not a problem. I
incrorectly assumed
it was calling strlen on the whole buffer because sscanf does exhibit the
problem I
describe, but as it turns out, the problem is inherent to sscanf, and
strtod works fine.

As a matter of fact, I just tested glibc's strtod on a very large ASCII
mmap'd buffer
just now, an it works fine, no quadratic behavior.

Apologies for not testing this better before reporting the bug. Please feel
free to close.

   - Emmanuel



On Tue, Aug 20, 2013 at 9:11 AM, neleai at seznam dot cz <
sourceware-bugzilla@sourceware.org> wrote:

> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
>
> --- Comment #1 from Ondrej Bilka <neleai at seznam dot cz> ---
> On Tue, Aug 20, 2013 at 02:12:32AM +0000, emogenet at gmail dot com wrote:
> > http://sourceware.org/bugzilla/show_bug.cgi?id=15854
> >
> >             Bug ID: 15854
> >            Summary: strtod should avoid calling strlen
> >            Product: glibc
> >            Version: 2.18
> >             Status: NEW
> >           Severity: enhancement
> >           Priority: P2
> >          Component: libc
> >           Assignee: unassigned at sourceware dot org
> >           Reporter: emogenet at gmail dot com
> >                 CC: drepper.fsp at gmail dot com
> >
> > Problem : glibc's strtod seem to systematically call strlen on its input.
> >
> > To the layman that I am, there doesn't seem to be any legitimate reason
> why it
> > should: it seems that strtod should simply consume its input one char at
> a time
> > until it reaches a char that marks the end of a valid FP number ASCII
> rep. and
> > should therefore work on a non-zero terminated buffer, as long said
> buffer ends
> > with a char that terminates the parsing.
> >
> This is not that big problem, strtod only uses strlen in following context
>
>   decimal = _NL_CURRENT (LC_NUMERIC, DECIMAL_POINT); // which is "."
>   decimal_len = strlen (decimal); // which is 1
>
>
> > This internal call to strlen makes it essentially impossible to call
> strtod
> > on a no zero terminated buffer, and there seems to be no other way to
> otherwise
> > access the non-trivial code that converts an ASCII buffer to a FP number.
> >
> > This makes it in particular painful to call strtod on a very large mmap'd
> > buffer of ASCII floats : strlen will plow through the entire file for
> every
> > call to strtod, making things highly inefficient (it is also not
> guaranteed
> > not to crash).
> >
> Do you have testcase to demonstrate quadratic behavior? It is possible
> that end is determined by other ineffective means.
>
> > To work around this shortcoming, one ends up having to figure out the
> end of
> > the FP ASCII string, "by hand", copy the result to a zero terminated
> buffer,
> > and then call strtod on that.
> >
> > This is both inefficient and clunky.
> >
> > See this article for a good description of the issue:
> >
> >
> http://www.ryanjuckett.com/programming/c-cplusplus/25-optimizing-atof-and-strtod
> >
> > Here's another instance of the problem:
> >
> >
> http://stackoverflow.com/questions/2033845/any-one-know-how-to-convert-a-huge-char-array-to-float-very-huge-array-perform
> >
> Not relevant for us as these are windows problems.
>
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> You are receiving this mail because:
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> You reported the bug.
>

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[Bug libc/15854] strtod should avoid calling strlen

[allan at archlinux dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

Allan McRae <allan at archlinux dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |allan at archlinux dot org
         Resolution|---                         |INVALID

--- Comment #3 from Allan McRae <allan at archlinux dot org> ---
Closing.

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[Bug libc/15854] strtod should avoid calling strlen

[neleai at seznam dot cz]

http://sourceware.org/bugzilla/show_bug.cgi?id=15854

--- Comment #4 from Ondrej Bilka <neleai at seznam dot cz> ---
On Tue, Aug 20, 2013 at 07:29:21AM +0000, emogenet at gmail dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
> 
> --- Comment #2 from emogenet at gmail dot com ---
> You are correct, the call tp strlen in strtod is not a problem. I
> incrorectly assumed
> it was calling strlen on the whole buffer because sscanf does exhibit the
> problem I
> describe, but as it turns out, the problem is inherent to sscanf, and
> strtod works fine.
> 
And could you provide sscanf testcase as separate bug report?

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Re: [Bug libc/15854] strtod should avoid calling strlen

[Ondřej Bílka]

On Tue, Aug 20, 2013 at 07:29:21AM +0000, emogenet at gmail dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15854
> 
> --- Comment #2 from emogenet at gmail dot com ---
> You are correct, the call tp strlen in strtod is not a problem. I
> incrorectly assumed
> it was calling strlen on the whole buffer because sscanf does exhibit the
> problem I
> describe, but as it turns out, the problem is inherent to sscanf, and
> strtod works fine.
> 
And could you provide sscanf testcase as separate bug report?

[Bug libc/15854] strtod should avoid calling strlen

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=15854

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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