Revision eacc6dfaea963ef61540abb31ad7829be5eff284 authored by Jens Axboe on 07 January 2020, 20:08:56 UTC, committed by Jens Axboe on 07 January 2020, 20:08:56 UTC
We currently punt any short read on a regular file to async context, but this fails if the short read is due to running into EOF. This is especially problematic since we only do the single prep for commands now, as we don't reset kiocb->ki_pos. This can result in a 4k read on a 1k file returning zero, as we detect the short read and then retry from async context. At the time of retry, the position is now 1k, and we end up reading nothing, and hence return 0. Instead of trying to patch around the fact that short reads can be legitimate and won't succeed in case of retry, remove the logic to punt a short read to async context. Simply return it. Signed-off-by: Jens Axboe <axboe@kernel.dk>
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syscall.c
// SPDX-License-Identifier: GPL-2.0
#include <linux/ptrace.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/export.h>
#include <asm/syscall.h>
static int collect_syscall(struct task_struct *target, struct syscall_info *info)
{
struct pt_regs *regs;
if (!try_get_task_stack(target)) {
/* Task has no stack, so the task isn't in a syscall. */
memset(info, 0, sizeof(*info));
info->data.nr = -1;
return 0;
}
regs = task_pt_regs(target);
if (unlikely(!regs)) {
put_task_stack(target);
return -EAGAIN;
}
info->sp = user_stack_pointer(regs);
info->data.instruction_pointer = instruction_pointer(regs);
info->data.nr = syscall_get_nr(target, regs);
if (info->data.nr != -1L)
syscall_get_arguments(target, regs,
(unsigned long *)&info->data.args[0]);
put_task_stack(target);
return 0;
}
/**
* task_current_syscall - Discover what a blocked task is doing.
* @target: thread to examine
* @info: structure with the following fields:
* .sp - filled with user stack pointer
* .data.nr - filled with system call number or -1
* .data.args - filled with @maxargs system call arguments
* .data.instruction_pointer - filled with user PC
*
* If @target is blocked in a system call, returns zero with @info.data.nr
* set to the the call's number and @info.data.args filled in with its
* arguments. Registers not used for system call arguments may not be available
* and it is not kosher to use &struct user_regset calls while the system
* call is still in progress. Note we may get this result if @target
* has finished its system call but not yet returned to user mode, such
* as when it's stopped for signal handling or syscall exit tracing.
*
* If @target is blocked in the kernel during a fault or exception,
* returns zero with *@info.data.nr set to -1 and does not fill in
* @info.data.args. If so, it's now safe to examine @target using
* &struct user_regset get() calls as long as we're sure @target won't return
* to user mode.
*
* Returns -%EAGAIN if @target does not remain blocked.
*/
int task_current_syscall(struct task_struct *target, struct syscall_info *info)
{
long state;
unsigned long ncsw;
if (target == current)
return collect_syscall(target, info);
state = target->state;
if (unlikely(!state))
return -EAGAIN;
ncsw = wait_task_inactive(target, state);
if (unlikely(!ncsw) ||
unlikely(collect_syscall(target, info)) ||
unlikely(wait_task_inactive(target, state) != ncsw))
return -EAGAIN;
return 0;
}
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