Messages in this thread | | | Subject | Re: [PATCH] vsscanf() in lib/vsprintf.c | From | Rasmus Villemoes <> | Date | Wed, 5 May 2021 16:35:52 +0200 |
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On 04/05/2021 21.19, Stefan Kanthak wrote: > Hi @ll, > > both <https://www.kernel.org/doc/htmldocs/kernel-api/API-sscanf.html> > and <https://www.kernel.org/doc/htmldocs/kernel-api/API-vsscanf.html> > are rather terse and fail to specify the supported arguments and their > conversion specifiers/modifiers. > > <https://www.kernel.org/doc/htmldocs/kernel-api/libc.html#id-1.4.3> > tells OTOH: > > | The behaviour of these functions may vary slightly from those > | defined by ANSI, and these deviations are noted in the text. > > There is but no text (see above) despite multiple deviations from > ANSI C > > <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/lib/vsprintf.c?h=v5.12> > > | /* '%*[' not yet supported, invalid format */ > ... > | /* > | * Warning: This implementation of the '[' conversion specifier > | * deviates from its glibc counterpart in the following ways: > ... > > More deviations (just from reading the source): > > 1. no support for %p
What on earth good would that do in the kernel?
> 2. no support for conversion modifiers j and t
Could be added, but do you have a user?
> 3. no support for multibyte characters and strings, i.e. %<width>c > and %<width>s may split UTF-8 codepoints
So what? The kernel doesn't do a lot of text processing and wchar_t stuff.
> 4. accepts %[<width>]<modifier>[c|s], but ignores all conversion > modifiers
Yeah, %ls is technically accepted and treated as %s, that's mostly for ease of parsing it seems. Do you have a use case where you'd want wchar_ts?
> 5. treats %<width><modifier>% (and combinations) as %%
What would you expect it to do? Seems to be a non-issue, gcc flags that nonsense just fine
vs.c: In function ‘v’: vs.c:5:18: warning: conversion lacks type at end of format [-Wformat=] 5 | x = sscanf(s, "%l% %d", &y); | ^ vs.c:5:20: warning: unknown conversion type character ‘ ’ in format [-Wformat=] 5 | x = sscanf(s, "%l% %d", &y); | ^
> 6. accepts %<width><modifier>n (and combinations)
Again, non-issue (warning: field width used with ‘%n’ gnu_scanf format)
> 7. doesn't scan the input for %[...]n
? What's that supposed to mean.
> 8. uses simple_strto[u]l for the conversion modifier z, i.e. assigns > uint32_t to size_t, resulting in truncation
Where do you see uint32_t? The code is
val.u = qualifier != 'L' ? simple_strtoul(str, &next, base) : simple_strtoull(str, &next, base);
case 'z': *va_arg(args, size_t *) = val.u; break;
so the conversion is done with simple_strtoul which return "unsigned long". And size_t is either a typedef for "unsigned long" or "unsigned int", so yes, of course a truncation may happen, but if the value actually fits in a size_t, it also fits in unsigned long (as returned from simple_strtoul) and unsigned long long (as stored in val.u).
Rasmus
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